The passage of a few years has given an extra tartness and distinction to Brian De Palma's first significant features, "Greetings" and "Hi, Mom'," recently booked together by Michael Clark of the American Film Institute Theater and now sharing the bill at the Circle through Monday.

These sarcastic, shaggy-dog satires about the state of the Union, callow youth division, were made back-to-back in 1968 and '69 and released by Sigma III, a small company owned by an up-and-coming film salesman named Frank Yablans. As it happens, De Palma's follow-up to "Carrie," an ambitious psychic thriller called "The Fury," scheduled to begin shooting outside Chicago next month, is being produced by Yablans.

Neither "Greetings" nor "Hi-Mom!" is consistently, unerringly funny; on the contrary, the level of comic inspiration and execution fluctuates from hilarious to dreadful. "Greetings" seems to have reached a wider audience, perhaps because the sequel took a precipitous plunge into racial satire. The second half of "Hi-Mom!," which begins on a disarming, giddy note of romantic comedy, is dominated by a long, grueling sequence designed to ridicule white liberals with an infinite appetite for guilt.

When the satirical touches are effective, these films have an unusually sharp, astute comic character. They seem more pertinent and durable than the other "underground" comedies of the period because De Palma, didn't limit his ridicule to the accepted Establishment targets. He regarded the counterculture and his own youthful protagonists as fair, equally preposterous game.

For example, "Greetings" begins and ends with President Johnson's memorable outburst of rhetoric during an address to the AFL-CIO: "I'm not saying that you've never had it so good - but isn't that the truth?" The movie takes a skeptical view of that remark, of course, but it tends to validate it in a satirical sense: De Palma doesn't find the best of all possible worlds, but he does find a volatile popular culture rich in comic possibilities.

De Palma is on such familiar, amiable terms with his young heroes, played by Robert De Niro, Gerrit Graham and Jonathan Warden, that he never seems to think of patronizing them, as Hollywood was about to patronize the restless or dissenting young.

If it's interesting to see De Palma's attitudes and techniques in a formative stage, it may be even more interesting to recall what De Niro was like at the beginning of his movie career, particularly on the eve of his latest picture, Martin Scorsese's "New York, New York."

De Niro comes into his own in "Hi, Mom!", recreating the same role he played in "Greetings" but suddenly expressing a vivid personality - or to be exact, a vivid split personality.