When the French Revolution passed from its first frenzy into a slightly less intense stage, it entered what historians have called the "Thermidor" period, named for the month in the French Revolutionary calendar when Robespierre fell. Judging by Playboy magazine, the sexual revolution has entered its Thermidor period.

The front cover of the September issue features a woman's face and not much else of her. She is demure enough for the cover of Popular Mechanics. But the back cover contains an ad featuring a crouching woman in a wet bathing suit. She stares out like a hungry cheetah ready to pounce over the large type that commands, "Light my Lucky." When the ads on Playboy's back cover carry more sexual wallop than the front cover, the magazine has entered middle age. Time was when Playboy's ethos communicated the message that middle age is a social disease.

In the 1950s the mere mention of Playboy produced sharp intakes of breath and pursing of lips. Today the magazine faces the tricky task of adjusting to altered values that it has been instrumental in altering. The supervisor of this tradition sits in a corner office filled with editorial clutter and overlooking Michigan Avenue's "Magnificent Mile." She is wearing a severe black suit, an electric pink blouse and a cameo brooch that looks Victorian. She is not. Victorian, that is.

Christie Hefner, 32, is slightly older than the magazine that generates $120 million of the $200 million revenues of Playboy Enterprises. Long ago Playboy's founding father decamped from the Chicago headquarters, going to earth in Los Angeles and taking up the arduous business of eating lotuses full-time. His daughter looks as though she dines exclusively on nouvelle-cuisine portions of whatever she eats. What there is of her slender self is fetching, but hers is not the, shall we say, pneumatic style of female form preferred by Playboy readers.

She wants Playboy to become a magazine people will put on the coffee table "next to Forbes." The domestication of Playboy may be evident in the fact that more people now get Playboy by subscription than buy it at newsstands.

She regards the magazine's rabbit logo tolerantly but dismissively as "institutional kitschiness." She hopes to give the magazine an ,elan suitable for people like her, who "grew up with the world Playboy made." Citing what Kinsey reported about Americans nearly 40 years ago, she argues that the sexual revolution has been more in attitudes than behavior.

In 1948, Kinsey, another Midwestern revolutionary (he taught at Indiana University in Bloomington), published the first volume of his "report." Writing with the cool immersion in detail appropriate to a zoologist, which he was, he showed that Americans were sexually . . . well, busy. He seems to have been startled that people were shocked by his statistician's tone of voice when he said things like: It is not surprising that some teen-agers are sexually active, considering that there are 450,000 instances of sexual intercourse among residents of Indiana each week.

But decorousness was in vogue at a time when The New York Times changed an advertisement for "naughty but nice" lingerie to "Paris-inspired -- but so nice." Four years later, Playboy came hopping along, and decorum has not been the same.

Playboy, says Christie Hefner, was from the start concerned with "whatever life style was called before we called it life style." We called it consumption, which Playboy has treated as a semi-erotic activity. But the most dated aspect of Playboy is its relentlessly liberal politics, which resembles the young Marx as misread by the old Mailer.

Two of the entrepreneurial brainstorms of the postwar world -- McDonald's and Playboy -- began in Chicago. Both are monuments to monomania -- Ray Kroc's focus on one kind of appetite, Hugh Hefner's on another. Why does Chicago loom so large as the incubator of marketing prodigies? Chicago grew to greatness as a railhead and as a butcher of animals that came by rail. Today it is a splendid blend of the elemental and elegant, with perhaps the nation's finest university, art museum and symphony. But McDonald's and Playboy pertain to the less delicate, more universal and insistent imperatives.

Playboy, with its leavening of literature, is like a city in that it has a high ratio of low to elevated concerns -- more commerce than cathedrals. To Hefner's credit, she disdains emulating those magazines that began in emulation of Playboy and now must try to titillate a jaded society that has millions of videocassette players pumping X-rated movies into living rooms.

Poor Penthouse, desperately seeking ways to disregard standards in a society where standards are thin on the ground. Iconoclasm is hard labor when you are ankle deep in the dust of smashed icons.