Big week for PBS

This past week may prove to be the highest-rated one in in PBS history.

According to early PBS estimates perhaps as many as 60 percent of all TV households in America watched PBS for an average of three hours per household. The week included ''The Flame Trees of Thika,'' ''Life on Earth,'' and ''American Playhouse,'' plus the premiere of the Granada/WNET-NY adaptation of Evelyn Waugh's novel ''Brideshead Revisited.''

Although PBS ratings reports are much more limited than those of commercial networks, it is estimated, based upon early overnight ratings from New York, that ''Brideshead'' will prove to be the highest-rated drama ever to air there. Some PBS shows in the top-rated group seemingly eclipsed by the apparent ''Brideshead'' ratings include ''Tinker Tailor,'' ''The Flame Trees of Thika,'' and ''Upstairs, Downstairs.'' It is probable that more than an ?average of 6 million households watched the premiere, which will be repeated in many cities. So it is possible that the totals will reach toward the 10 million mark in the long run, although PBS officials decline to estimate what the totals may be for the entire 11-episode run.

''Brideshead Revisited,'' an adaptation of Evelyn Waugh's 1945 novel of the same name, deals with obsession, repression, guilt, and redemption. The film series, like the novel, deals with class and eccentricity in pre-World War II British society, and expounds at length on redemption through Roman Catholicism. Some viewers may find objectionable the dramatization of what appears to be flagrant homosexuality, alcoholism, and immoral behavior, although there is final retribution by the end of the drama for most of those involved. The TV version, which follows the book faithfully, is a beautifully mounted and acted production, with many scenes shot in and around Castle Howard in Yorkshire, which serves as brideshead.