Once you know that a playwright has chosen a hospital ward as his basic image of society, you have a fair idea of what the playwright is going to tell you. So, if you don't want to be told that society (particularly British society) is sick, "The National Health," now playing at Arena Stage, may not be your cup of tea. But playwright Peter Nichols delivers his diagnois with such unexpected twists, curious angles of approach and comically baroque ramifications that even a sick society should recognize him as a very robust talent.

The play's clearest attraction in the cleverly staged Arena production is its fast-paced, bawdy-and-blood hospital humor, in a style familiar from the British movie "Carry On, Nurse" and its sequels. You can buy a ticket just for that and find your money's worth. But beyond this and its more serious hospital-as-microcosm dimension, it has quite a few charms that are peculiarly its own: a brilliant pantomine-ballet of a kidney transplant operation, for example; a lot of true-life melodrama presented with the muted organ music of daytime television panting in the background; and a grisly-comic running commentary on the facts of life and (perhaps more important) death in our time.

Nichols is a very funny writer, in styles that range from slapstick to the subtlest of irony, but his wit never allows you to forget that he has something serious to say. His analysis of what is wrong with his native England is so thorough and so cutting that it may make us glad to be Americans - if only because we are less likely to see him dissect us onstage.

The Arena production's cast is large, very able and superbly balanced in its interpretation of an unusually tricky script. At first, I thought the show was being stolen in this production by the smooth and quietly sardonic talent of Jarlath Conroy, but by the final curtain (or rather the final blackout, since this is theater in the round) it became clear that Nichols wantted it that way.

Barnet, the orderly played by Conroy, has no real place in the plot, such as it is, but Nichols gives him some of the show's best lines in a series of detached little mono logues. He comes on periodically, like a Greek chorus, to speack directly to the audience on what is happening or to give horribly charming and (against all probability) funny little talks on his work - handling a new corpse, shaving a patient for surgery. and similar pleasantries. The surface subject doesn't matter; what Barnet is talking about is the human condition, and his basic attitude (reflecting his creator's) is that you might as well laugh at it because crying doesn't help.

Among the patients who supply most of the play's action, only one - Ash, a disgruntled clerk and failed schoolteacher well played by John Wylie - emerges as a fairly sympathetic human being. The others are standard types: Rees (Leonardo Cimino), an old doctor getting some of his own medicine and hating it; Mackie (Robert Prosky), a terminal cancer patient who thinks mass euthanasia is the solution to England's problems; Loach (Stanley Anderson), an aloholic cockney who lives mainly on his shaky pride and nostalgia for the lost Empire.

When he is not finding and exposing the hidden flaws in various types of English character, Nichols takes substantial swings at politics ;socialist and Conservative), religion (organized and disorganized), social and racial pretensions and even true love.

If this summary makes "The National Health" sound like a rather mixed bag . . . well, so it is. But it is a mixed bag containing a lot of miscellaneous goodies, and I enjoyed it thoroughly.