FEW taboos exist in the blood-and-gore world of shoot-'em-up video games. But game makers have traditionally respected one rule: no Nazi protagonists.

Last year that rule was challenged on at least two fronts. In November, Activision released Return to Castle Wolfenstein, a game in which players take the role of a United States soldier on a mission ''to thwart Heinrich Himmler's occult and genetic experiments.'' The multiplayer version, which pits players against one another online, allows some players to fight as German soldiers.

And even before Return to Castle Wolfenstein, another realistic first-person shooter with a Nazi protagonist was making a stir: Day of Defeat, which was released for online play last January.

Unlike Castle Wolfenstein, Day of Defeat was not issued by a major game maker. It is a modification of the popular science-fiction PC game Half-Life, created by 17 young programmers writing code in their spare time.

Todd Hollenshead, chief executive of Id Software, which produced Return to Castle Wolfenstein for Activision, described the games as a result of the renewed interest in World War II evidenced by films like ''Saving Private Ryan'' and ''Enemy at the Gates.''

''The trend you're seeing with new games is, to some extent, a reflection of what's going in the culture,'' Mr. Hollenshead said. ''For instance, you've now got games with terrorists and counterterrorists. And World War II games such as Return to Castle Wolfenstein and Day of Defeat reflect what you see in popular movies.''

Yet Day of Defeat not only shows battlefields decorated with swastikas and Nazi posters but also attracts many players with an enthusiasm for neo-Nazi role playing. The game tries to recreate specific World War II battles. Soldiers on the German side wield Gewehr 43 semiautomatic rifles, Luger pistols, so-called potato-masher grenades and, when the ammunition runs out, Hitler Youth knives. In some games, a battle is signaled with a rousing call to arms broadcast in German.

The presence of swastikas and other Nazi symbols is so pervasive that the game might be viewed as illegal in Germany, where the dissemination of Nazi thought and symbols is banned. The programmers are moving to avoid any trouble in Europe.

''We're working on a version that won't present problems,'' said Matt Boone, who led Day of Defeat's programming team. ''We want to make sure Germans can play the game and not break the rules.''

The chat rooms that combatants use to type messages to one another during games have also become battlefields of a sort, with players using Nazi-inspired names and expressing neo-Nazi sentiments. A recent scan of active Day of Defeat game servers revealed a list of Axis players with noms de guerre like Mein Kampf, HitlerYouth and ZyklonB, although it is hard to determine whether the names were chosen simply to get attention. In a recent game, a player on the German side using the name AnneFrank was questioned about that moniker. ''Oh, you know,'' the player said. ''I'm just trying to keep things spicy.''

Some groups of players called clans who routinely play Day of Defeat together also refer to themselves with Nazi labels. A well-organized group of German students based in Singapore calls itself Clan SS, a reference to the ruthless Nazi military divisions responsible for some of the war's worst atrocities. ''Yes, German,'' said Lorien Stoetzel, who identifies himself online as the clan's ''Oberfeldwebel,'' meaning platoon sergeant. ''But we're not neo-Nazi freaks. Everyone asks me that, but we're not.''

Mr. Stoetzel said that the group's members simply enjoyed playing games and had chosen the name because they found it provocative.

Some gamers contend that tasteless provocation is a staple of the gaming world as a whole. ''It's a problem in all games, unfortunately,'' said Bruce Boyden, 34, a Washington lawyer and self-described 10-hour-a-week gamer. ''There's always the odd hateful taunt. Sometimes it goes beyond a single word or two, and somebody starts ranting about their racist viewpoint nonstop. The problem is that the majority of players are white males in their teens or early 20's, so the dynamic is about the same as in any boys' locker room.''

Mark Weitzman, the national director of the Simon Wiesenthal Center's Task Force Against Hate, based in New York, said he found the use of Nazi protagonists disturbing. ''It encourages people to express what are rightly considered to be socially unacceptable sentiments -- racism and anti-Semitism and hate,'' he said.

Alex Forman, a 32-year-old gamer from Yorkshire, England, was disturbed enough by the introduction of German protagonists that he wrote and posted an essay entitled ''Return to Castle Wolfenstein: What About the Holocaust?''

In an interview, he said of Day of Defeat: ''The historical angle certainly enhances the game play and lends itself well to the whole 'kill or be killed' atmosphere. But you have to wonder if the specific overtones in the game were really necessary.''

The programmers behind Day of Defeat contend that they were interested in ensuring historical accuracy in the equipment, weapons and uniforms, not in providing an outlet for neo-Nazi sentiment.

''There's nothing like traveling back more than half a century to put yourself in the boots of a World War II soldier storming the beaches at Normandy,'' said Matthew Lane, 17, a Day of Defeat Web designer from North Carolina. ''As kids, many of us have dreamt what our grandfathers and fathers suffered through, and fought for, more than 50 years ago. Day of Defeat just brings these things to reality.''

The creators of Castle Wolfenstein and Day of Defeat point out that the soldiers depicted are regular German soldiers, not SS troops. (In the case of Day of Defeat, there are already tools to hack the graphics features to change uniforms. An Austrian clan called Sonderkommando Carinthia has posted a downloadable file that causes a German-side Day of Defeat player to appear in the uniform of an SS soldier.)

The makers of Return to Castle Wolfenstein say they sought to avoid Nazi symbolism that would promote dark role-playing. ''I wouldn't say that we believe we have to be politically correct about this sort of thing,'' Mr. Hollenshead said. ''But we definitely designed the game to be more Axis versus Allies than Nazis versus Allies.''

He added that the game lacked the Nazi symbols that might make it illegal to sell in Germany. ''The German market is one we believe is significant,'' he said.

The multiplayer version of the game has no Nazi symbols, but the single-player version does include Nazi imagery. And the Web site says the game cannot be sold in Germany or Austria.

So far, there is little evidence of Nazism, whether genuine or postured, on Return to Castle Wolfenstein's online servers. Still, the game's creators say that the presence of Axis protagonists is bound to attract criticism.

''I don't doubt there are going to be people that go out and distort what the multiplayer gaming experience is and say, 'Oh, I can't believe you guys did this,'' Mr. Hollenshead said. ''There are a lot of critics of the game industry, and they look for things to criticize.''

For some gamers, many of the debates are resolved in the game itself.

''Most of the time I play, there is at least one guy on the server with a name like Dr. Goebbels or something,'' said Chris Oakley, 34, a Day of Defeat player from Montreal. ''I egg them on by typing like 'Long live Zionism.' I always get a rise out of them. They go out of their way to hunt me down.''

Photos: SPECTERS -- Day of Defeat, lower left and top left, was the first video game with a Nazi protagonist. Castle Wolfenstein features German soldiers, but its makers say they sought to avoid encouraging dark role-playing.