The path of true love runs uncommonly rough in A. R. Gurney Jr.'s engaging new comic romance. The observation is thoughtful and pertinent. The structure is compact. The performances are adroit in the production newly arrived at the Theatre at Saint Peter's Church in the Citicorp Building.
The action of ''The Middle Ages'' takes place in the trophy room of a sedate men's club in a large city and spans the era from the mid-1940s to the late '70 s.
The plot concerns two long-term relationships. On the one hand there is the exhibitionistic rebellion of scapegrace Barney Rusher against the tradition represented by his austere father. On the other hand, there is Barney's steadfast but unrewarded devotion to conventional Eleanor Gilbert. After an opening scene that reunites Barney and Eleanor at the funeral of Rusher pere, ''The Middle Ages'' returns to the days when Eleanor was the cute new girl in town and Barney was launching his career as a one-man shock brigade assaulting the bourgeoisie.
Eleanor takes out the approved domestic insurance by marrying Barney's buttoned-down younger brother. In the course of time, Barney's widowed father weds Eleanor's divorcee mother. Meanwhile, Barney embarks on a careening career. He becomes a medieval scholar with a PhD degree, has a short but profitable career making pornographic movies, and plunges into the sexual revolution. The constants in his hectic existence are his love for Eleanor and an unadmitted devotion to his father.
''The Middle Ages'' refers both to the chivalric era with which Barney eccentrically identifies, like some Don Quixote on a noisy motor bike, and to the time of life when a sadder and wiser Barney encounters the now matronly Eleanor. Mr. Gurney is concerned not only with whether Barney and Eleanor will find themselves and each other but whether Barney and his father will reconcile. Barney is at the same time a rebel with several causes and a contemporary figure of the prodigal son.
The club trophy room - with its secular icons - is a natural setting for the brief encounters of ''The Middle Ages.'' Mr. Gurney has used it to frame his literately witty and good-natured satire on the rituals, customs, and shibboleths of a smug society where conformity reigns and nonconformity is the discomfiting sin. The comedy itself is traditional, even old-fashioned. But the approach is characteristically Mr. Gurney's as he touches in passing on such phenomena as racism, the Korean war, Berkeley campus activism, the Nixon era, women's rights, and divorce.
As Barney and Eleanor, Jack Gilpin and the delectable Ann McDonough bring the light touch and the underlying serious tone essential to a comedy that seeks to look below the surfaces. They are quite marvelous at catching the changing attitudes of changing times. Jo Henderson bustles amusingly as the self-propelled divorcee for whom opportunity will not need to knock twice. Andre Gregory's portrait of Barney's father seems more fastidiously Brahmin than energetically Middle American. Nevertheless, the performance staged by David Trainer is by turns as deft and broad as the script requires.