There is nothing Paris loves better than a pretty girl, so when the billboards went up around town people began to buzz. There she was in 10-foot-high living color -- a bikinied brunette shining with health and glistening with the sea water that pearled on her little limbs.

"On Sept. 2, I will take off the top," she proclaimed in bold black letters.

That was Aug. 31. Three days later, right on schedule, she kept her promise. The green bikini top was shed and she smiled provocatively down on passing Parisians with her breasts in full view. Even more glorious was the pledge, in those same imperative letters:

"On Sept. 4, I will take off the bottom."

At that, the buzz rose in volume. Newspapers took interest. Questions went out. Speculation and jokes ricocheted across the little table tops of sidewalk cafes and the fine linen of elegant restaurants: Will she? How could she? Why would she?

Before dawn on the appointed day, sign-pasting teams rolled through the streets and Parisians got their answer -- she did. The bottom was gone, all right. The girl had turned her back to the photographer, but she stood there stark naked, just as she said she would.

This time the black letters said: "Avenir, the billboard company that keeps its promises."

The feminist groups complained. The French Advertising Verification Bureau condemned. And the new government reminded one and all it stands for women's rights. But many Parisians seemed glad for the diversion just as they were returning from summer vacations to what other signs increasingly point to as a difficult year ahead.

"I HAVE NEVER seen such an anguished rentree," said a French lawyer, admittedly no partisan of the government. "In my business I see a lot of businessmen, and they are really worried."

La rentree is when Frenchmen, whatever their station, return to work after an August at the beach. Fathers go back to the office or factory, tanned and many thinking already of the next vacation. Mothers take their children to buy school clothes and supplies, featuring this time, for pupils who have to chew to think, banana-flavored pencils.

Traffic suddenly thickens and the easy summer pace speeds up. Neighborhood laundries have reopened. Once again the plumber answers his telephone.

France, although perhaps not as "anguished" as the lawyer's conservative businessman friends made out, does seem ready to be serious.

This year, however, a new doubt has entered the picture. Last May French voters elected Francois Mitterrand, a Socialist, as their president. And now -- la rentree -- his government is set to enact the legislation it says will enable it to keep campaign promises of reducing unemployment, slowing inflation and increasing social equality.

The Socialist plans center on a limited nationalization program, increased taxes on the wealthy and expanding the job market by creating about 200,000 new positions through government spending.

In newspaper articles, opinion polls and conversations, Frenchmen are beginning to ask, "Will it work?" After the summer honeymoon -- here it was called a "state of grace" -- they have noticed that since the May 10 election inflation has risen from 12.5 percent to 13.5 percent and that unemployment climbed from 1.6 million to 1.8 million.

SOME MEMBERS of the defeated government have begun to speak up with warnings about what they say will be the effects of Mitterrand's policies. The still-popular former health minister, Simone Veil, said the other day she is worried about the future of France. The dethroned president, Valery Giscard d'Estaing, has held his tongue, however, reportedly against his own instinct and on the advice of friends counseling that the stigma of defeat is still too vivid for his words to carry the proper weight.

The French public, according to recent polls, appears to be willing to go along for the time being with Mitterrand's program, but the signs of worry have emerged.

The percentage of those who are confident he will lead them well dropped from 71 to 62 between July and September, according to a poll for the conservative Figaro Sunday magazine. Similar drops were recorded for confidence in his ability to control prices and reduce unemployment.

THE PROMISE of difficult times has not visibly dimmed spirits in the government and visitors to ministerial offices find they are breathing a fresh new air of adventure and experiment.

A caller to Prime Minister Pierre Mauroy's headquarters, for example, was treated to informal slang from his aides and casually shook hands with a Cabinet minister in the parking lot on the way to lunch. For a correspondent whose previous experience in France was under the buttoned-down Giscard government, it was a refreshing change.

The change was symbolized two weeks ago by the president himself. An Israeli kibbutznik who had met Mitterrand during one of the Socialist leader's trips to Israel and received the inevitable, "Look me up when you get to Paris," actually did. He telephoned the Elysee Palace and asked for Mitterrand. Whereupon the president came on the line and invited him over to lunch.