A picture of a scantily-clad go-go girl leaps out of the ad in a 1978 guide for visitors.

"Tehran's most sensational and exotic nightclub," the ad proclaims "brings you into the gay world of Paris. Dance till dawn to the fabulous Pino de Martis band."

Today, there is no dancing till dawn in the nightclubs and discotheques of Tehran. All have long since been burned down or closed.

There are no more scantily clad women, even on beaches along the Caspian Sea. Under the recent proclaimed Islamic republic, beaches now are segregated by sex.

In fact, gone are most of the things advertised in the old brochure, from the operas and ballets at Roudaki Hall to Scotch whiskey, which can be bought only at $70 a bottle on the black market now.

These are some of the features of the new Iran that might not appeal to foreign visitors and rich, Westernized Iranians. But for the average citizen, it hardly matters. Most of those Western diversions were financially out of reach anyway.

What counts for most Iranians one talks to is a sense of liberation after the government of Shah Mohammad Reza Pahlavi was swept away two months ago. They do not seem terribly worried about the country's economic problems and the rather undemocratic outlook of the new powers-that-be. Life, at least for now, is freer than it used to be, and few people long for the old days.

The new sense of freedom is evident when one walks through the streets of Tehran. Knots of people gather around the latest wallposters. In some places the posters, which espouse varied points of view, spark impromptu sidewalk debates. All kinds of literature are available at sidewalk stalls: from Islamic works to Marxist manifestoes, from portraits of the Ayatollah Ruhollah Khomeini to pictures of Che Guevara.

Foreigners seldom are harassed on the streets of the capital now as they were during the struggle against the shah. After the evacuation of thousands of foreigners in recent months, the general feeling seems to be that if one is still here, somehow one must be all right.

Even the inherent ugliness of Tehran - an architectural nightmare and city-planning disaster of hodgepodge buildings, prefabricated overpasses and traffic-congested streets - seems easier to take these spring days.Treelined avenues camouflage the motley facades, and breezes waft away some of the smog, affording a better view to the north of the snowcapped Alborz Mountains, the capital's only scenic attribute.

How long Iran's so-called revolutionary spring will last is debatable. For a fair number of Iranians one encounters, it already is not without disappointments.

"Nothing has happened to make me wish the movement had not taken place or make me want to go back to the old days of SAVAK," the shah's secret police, said a 50-year-old professor.

"I only hope the present and future government will let people be free to think and say what they like. Right now there are signs that give you hope and signs that make you fear."

He said that when he walks down the street, Islamic fanatics occasionally shout at him to cut hi drooping mustache. (according to Islamic teachings, the hair of a moustache should not hang over the lip). In his classroom, he said, some radical students accuse him of having "cooperated" with the old government by having taught under it. He is worried that good professors may be forced out of their jobs, or worse, because of things they said when they had to be constantly wary of SAVAK informers in their classrooms.

He says he voted for an Islamic republic in last month's referendum "to end the old system," although he admits he has "no clear ideas" what an Islamic republic is. He says he would rather have had a wider choice than the one between the monarchy and the Islamic republic.

In an interview before the revolution he didn't worant to be named. He still dooesn't, and he suspects that Khomeini's committee, is tapping telephones just as SAVAK used to.

A female bank worker has no such reservations about an Islamic republic, although she abhors the chador, or full-length veil, traditionally worn by devout Moslem women here.

She claims that Khomeini's controversial statements about hejab , which can be translated as wearing the veil or modesty, and which caused a series of angry women's marches, were exaggerated and misunderstood.

"Hejab is within you," she says. Though not religious herself, she asserts that "Islam is the only thing that could have united the people against the shah."

An Iranian journalist, also a woman, sares her compatriot's indifference to the chador and religion, but is much less tolerant of the new Islamic government. Faced with only a choice between that and a monarchy, she voted in the referendum for the Islamic republic.

"What else?" she says.

She would have boycotted the referendum entirely except that a seal of participation was stamped on identity papers, and she was afraid she might not be permitted to leave the country in the future if she could not produce it.

Here are some other items on life in the Islamic republic:

The government is sending a delegation to Australia and New Zealand shortly to verify their methods of slaughtering cattle and sheep. The issue arose when Khomeini banned frozen meat imported from the two countries as "forbidden" because it was not slaughtered according to Islamic practice.

The meat should be thoroughly bled before freezing. The Iranian Meat Organization released frozen meat stocks after seeing films of the slaughtering process presented by Australian and New Zealand representatives. A meat organization official then said Khomeini only "thought" the meat was forbidden. Anyway the delegation is to include two meat organization officials, two butchers and two Moslem leaders.

Another hot topic these days is seegheh, the Iranian practice of temporary marriage. Introduced during the times when merchants went off for months or even years on trade caravans across continents, the practice allowed men to contract for feminine companionship for a certain length of time while leaving their wives and children at home.

The man would choose a suitable companion-usually a nubile young village girl-and make a deal with her usually destitute father with the approval of a local religious leader. At the end of the agreed time a divorce would be declared and the erstwhile husband would pay off.

This Shiite Moslem practice suited the man and the girl's father, who stood to make some needed cash. In essence a rather complicated form of prolonged prostitution, seegheh marriages still exist, although they were rare.

The question now is whether the practice can continue under the Islamic republic. The verdict so far, according to the government spokesman: "It is correct that men can obtain it." He added that religious leaders would have to study the separate issue of polygamy, banned under the shah but permitted by Islam.

The brevity of bikinis on Iran's Caspian Sea beaches once rivaled Europe. And no one seemed to mind when more traditional women waded into the water fully clothed, sometimes even wearing their chadors.

Now, however, the military commander of Bandar Anzali has announced that in accordance with Islamic practices, there will henceforth be separate bathing facilities for men and women.

The good news is that this is an improvement since women previously were not allowed to bathe publicly in that town at all. The bad news is that the arrangement is expected to spread to other towns that previously were more liberal.

As far as it known, the Islamic status of bikinis hasn't been tested, but the prospects for Iranian girl-watchers this summer look grim.

Some enterprising revolutionaries have found a new way to vent public hatred for the shah and the American government which supported him for so long. Sidewalk shooting galleries have sprung up, allowing the vengeful to pepper pictures of the shah and President Carter with pellet guns. It is hard to tell which one is the more frequent target. Khomeini's guards stationed at the American Embassy following a Valentine's Day guerrilla attack on it have taken to interrogating visitors, such as journalists, who arrive for appointments with embassy officials.

A fat, bearded guerilla seated in a gatehouse adorned with posters of Khomeini and Palestinian leader Yasser Arafat usually tries to do this in Persian, with one of his comrades interpreting in broken English. The visitors are grilled about whom they want to see and why.

National Iranian Radio and Television chief Sadegh Ghotbzadeh plans to cut transmissions to 21/2 to three hours a day instead of the four to five at present. He wants television, which the government censors, to concentrate on "programs of guidance" with some "general entertainment" thrown in, but in leser doses.

The revolution's leaders appear to have reached the conclusion that excessive TV-watching detracts from conversation between children and adults.

This box, therefore, destroys the human relations of the family," Ghotbzadeh says.

He adds that a "limited minority" keeps calling up to ask a halt to endless televised discussions of Islamic problems and the return of the old programs, which included "Kojak," Gunsmoke," "Peyton Place" and others.

"of course this we shall not do," he says, though old American silent comedies somehow have been deemed sufficiently Islamic, or revolutionary.