A theatrical event of surpassing delights is taking place in the tiny loft theater atop Playwrights Horizons on West 42nd Street. Entitled ''The Dining Room,'' it reassembles in a flow of flashbacks the several generations of families, friends, and domestics who have converged on an area once considered requisite for any self-respecting, middle-class home. In his series of thematically orchestrated vignettes, playwright A.R. Gurney Jr. demonstrates the dramas that dining room walls might reveal if they could talk.
''The Dining Room'' begins as a real estate agent (Lois de Banzie) exposes the room's advantages to a very contemporary client (John Shea) who regards it as a tempting but resistible anachronism. No sooner have they departed than the room is taken over by a stuffy disciplinarian (Remak Ramsay), his docile daughter (Pippa Pearthree), and a son (W.H. Macy) who asks intelligently awkward questions.
''Half of life is learning to meet people,'' says the pontifical parent.
''What's the other half, Dad?'' his little son asks ingenuously.
And so it goes as meals are set and cleared, voices raised and lowered, crises confronted, and a stream of familial matters subjected to Mr. Gurney's sympathetic and humorous scrutiny. Six expert actors play multiple roles in the 18 episodes of the Gurney journey through time. The dining room, with its elegant but unostentatious Hepplewhite furniture, becomes a microcosmic setting for the seven ages of man, woman, and child in Northeast WASP America. The players accomplish all of their characterizations with no change of makeup, thus adding to the pleasure and wonderment of ''The Dining Room.''
The whole is the sum of its intricate parts and defies any attempt at detailed description. Of many but fleeting impressions, a few must serve to indicate the nature of the performance: Mr. Ramsay as a small boy and a crusty but indulgent patriarch; Mr. Shea as a psychiatrist and Mr. Macy as the architect who would like to chop up the dining room; Miss Pearthree ranging the gamut from tots to octogenarian; Miss de Banzie in a variety of matronly and other roles; Ann McDonough as a blase teen-ager and a faithful but departing maid. (There are few servant problems in ''The Dining Room.'')
Credit David Trainer for a smoothly flowing performance. The simple but handsome set is by Loren Sherman, lighting by Frances Aronson, and costumes by Deborah Shore.