Washington — In Washington, D.C., the Korean war comes on at 7:30 p.m., five nights a week - when reruns of the TV series ''M*A*S*H'' are shown on a local station. For those who prefer a different type of humor, ''Three's Company'' airs at 7 o'clock, competing with network news. ''The Jeffersons'' are on twice an evening: at 6 p.m., and again at 6:30.
All across the nation the airwaves are flooded with reruns during late afternoon and early prime time. These aged network hits are not broadcast this often because they are necessarily great art. They are on because they still attract large audiences.
Reruns mean big bucks - and a fierce lobbyists' war now raging in Washington centers on who should get a share of this money. On one side are Hollywood producers, on the other the ''big three'' networks. Both sides say that if the other wins, free TV will become even more of a wasteland than it already is.
Think of the whole thing as a miniseries called ''The Winds of Deregulation.'' At issue are the syndication and financial-interest rules, Federal Communications Commission (FCC) regulations adopted in 1970 that prevent TV networks from investing in shows they don't produce themselves and from owning syndication rights.
NBC, for example, doesn't own ''Hill Street Blues.'' The network rents the right to show each episode twice. MTM Enterprises, the company that actually films the series, retains ownership and at some point will probably syndicate it - rent old episodes to individual stations.
Networks were barred from the $800 million-a-year syndication business because the FCC feared it would give them too much power over program production. But the commission began to take another look at the rules in 1977. This fall, the FCC proposed letting the networks gradually back into syndication , concluding in a special study that the prohibitions were ''misguided at best.''
But Hollywood producers find the FCC proposals as appalling as snow on Sunset Boulevard. They say cash they earn from syndicating their shows is a key to their survival - and they fear a network could force them to sell off syndication rights when it buys their shows.
Critics of the FCC proposal also are concerned that networks would squeeze out independent stations by refusing them their most popular drawing card - network reruns. Networks, with more direct control over programming, would force production of even blander fare than what is now aired, they say.
''The history of the networks has been to favor sameness, to go for the least offensive programming,'' grumbles Andrew Schwartzman, director of the public-interest group Media Access Project.
Network executives retort that there's no reason to believe they will corner the market on reruns, since they didn't before the rules took effect in 1970. CBS vice-president Roger Colloff notes that the rerun champ, ''M*A*S*H,'' is still owned by 20th Century Fox, even though it began before 1970 and CBS tried hard to buy syndication rights.
Without syndication revenue, the networks say, they won't be able to compete with cable and other new video technologies, and free TV will languish.
FCC commissioners could vote ''any day now'' on whether to go ahead with their proposal to loosen syndication rules, says a commission spokesman. But the Senate Appropriations Committee has voted to block FCC action for six months. Several bills have been introduced to block action for five years. And President Reagan, a former Hollywood actor, is becoming interested; he was briefed on the subject last week.