AS ANY GUIDEBOOK will tell you, Leningrad is one of the most beautiful cities in the world. At the end of a fully-packed three-day tour, our little group agreed not only that Leningrad is beautiful, but that it must have been breathtaking when the "Venice of the North," built on dozens of islands, was the showcase of the czars.

The group to which I refer consisted of 29 who, after attending the International Monetary Fund/World Bank meetings in Helsinki last month, seized the opportunity to visit the Soviet Union.

Leningrad is a city of 5 million. Its broad boulevards and huge squares still reflect the style and talent of the European architects and artists employed by Peter the Great to create his capital on the Baltic.

To be sure, there is a grimmer side of the city that one senses but cannot document on a brief trip as a tourist: one spots the inevitable queues for every simple necessity of life, the sober faces, and the dull monotony of the monolithic apartment towers that dot the landscape, especially on the city's outskirts.

The Russians one encounters as a tourist--guides, waiters, salespersons in the shops--are routinely pleasant. But elsewhere, the Russians present an impassive and unsmiling visage. A single exception: the energetic ushers at the ballet and opera, mostly strong and bustling women, who went out of their way to help us find seats--even squeezing in chairs where they didn't belong.

Language, of course, is a great barrier. We had an enormous advantage in that one of the group, David Armour of the IMF, was not only fluent in Russian--but also knew Russian history and art. (Armour, who does the Morning Press summary for the IMF, even gave us a daily two or three-paragraph fill-in of the "news" each day, after combing through Pravda and Izvestia. He was also a handy man to have around at the Russian opera.)

We each paid roughly $300 (slightly more for single occupancy), which included round-trip air transportation from Helsinki, three nights and full board in a comfortable, new Finnish-built hotel, and most tours of the city and museums. Extras (but very cheap) included ballet and opera tickets.

Less than an hour by air from Helsinki, Leningrad is a holiday magnet for the Finns, who come in large numbers by air or by boat across the Gulf of Finland. A big attraction for Finnish tourists appears to be the low-priced vodka and other liquor that is sold for Finn-marks or other hard currency. Finnish teen-agers--who are steered out of the Helsinki dance halls at 6 p.m.--dominate the scene in the hard-currency hotel bars in Leningrad.

The Leningrad that the tourist is shown is essentially a celebration of the revolution: the buildings, monuments, and public squares that highlight the "Great October Socialist Revolution" of 19l7. Private ownership of homes is theoretically outlawed, but we were shown a few houses outside the city on the route to Peterhof that were said to be individually owned.

Peterhof (also called Petrodvorets), is the restored summer palace just outside of Leningrad, on the coast of the Gulf of Finland. Obviously patterned after Versailles, it was virtually destroyed during the German occupation of 1941-44, and then painstakingly rebuilt by the Soviets. Many original pieces that had been hidden from the Germans are now back in Peterhof, and overall, the restoration is extraordinary--although the fountains touted by the guidebooks weren't working when we were there. If you have seen 18th- and 19th-century furnishings elsewhere, you can cut short the tour through the palace itself, allowing time to wander through the gardens and get a look at the Baltic Sea.

But above all, there is the Hermitage, which may be the world's greatest art museum. For me a trip to Leningrad is worth the price just for the visit to the Hermitage. It is breathtaking. The Hermitage now occupies five buildings, one of which was the Baroque style winter palace built by Rastrelli in 1796. The huge complex of buildings, on the banks of the Neva River, contains not only a sampling of the excessive wealth of the Russian royal family, but probably the greatest collection of French Impressionists assembled anywhere--room after room of Matisse, Cezanne, Degas, Gauguin, Renoir, Pissarro and others. Most of these have never been exhibited in the West. Three priceless large Kandinskys are almost hidden on a stairhall landing.

The Hermitage is visited by 3 1/2 million persons a year, the Russians say. It was just recently announced in Moscow that the exhibits will be spread out over three additional, nearby buildings beginning later this year.

By the way, carefully check as soon as you get to Leningrad to make sure that the Hermitage is open on the day you're scheduled to go: when we were there, it was closed on Mondays. But the Intourist people (the Soviet travel agency) are flexible. Insist on revising your schedule if necessary to make sure that you get to the Hermitage--anything else can be sacrificed for it.

In the Hermitage, try to break away from your guide who will want to concentrate on the "gold" museum (a display of the golden excesses of the czars and czarinas) and on Russian art and find your way to the third floor where the French Impressionists are displayed. Otherwise, you will be hustled through the Impressionists in the final 10 minutes of the tour. (For Russian art, most tours incorporate a separate visit to the Russian Museum, with its priceless icons.)

Another tourist stop is the Peter and Paul Fortress, where some of the leaders of the revolution were kept in solitary confinement. Inside the fortress stands the St. Peter and St. Paul Cathedral. Its golden spire is visible from most parts of the city.

But despite the visibility of the monuments and public buildings, there was always the knowledge that the tourist sees only that part of the city that is "on show." A newly-published Bantam Book guide to prisons and concentration camps of the Soviet Union indicates there are 18 penal facilities in or near Leningrad, including 11 psychiatric prisons in the city itself. You will not see them on a tour.

This is not to say that you can't poke around on your own, and wander a bit around the city, especially along the main drag, the Nevsky Prospekt. A very efficient Metro charges 5 kopecks--roughly 7 cents to go anywhere in the system. Good city maps are available, including a Metro Plan, in the hotels.

Almost uniformly, our group felt a sense of exhiliration at the end of the trip. For most of us, it would probably be our only visit to the Soviet Union, and we felt we got a glimpse of where important history had been made. We saw a great deal in three days, although we were sometimes too rushed, and the Intourist planning less than satisfactory. On the road to Peterhof we obviously took a roundabout route so that the guide could collect or deliver something to his office. Nonetheless, Sascha was an earnest and likeable university student who faithfully delivered the party line on all occasions.

The guide who came along from Helsinki, however, was inexperienced. Although she undertook to arrange for ballet and opera tickets, she waited until the last minute to do so. Thus, one night at 7:10 while still trying to throw down dinner, we obtained tickets for "Esmerelda," to be performed by a Kirov company at 7:30. Since the promised bus did not appear, some 16 of us scouted for cabs (there was no taxi stand at the hotel) and worked our way back later via Metro. Of course, we missed part of the first act. But it was a great performance, and something of an adventure.

Another problem was our hotel, which was 30 minutes by bus from the city, shorter by Metro. We wasted time getting back and forth for meals since most of them were paid for. (Some eventually gave up their pre-paid lunch to stay on in town). The trade-off is that the in-town hotels, like the well-known Leningrad, are not as new or well-equipped, and, according to other IMF/Bank people, not as clean or warm.

When you register at your hotel, you probably will be given a card along with your room key. Keep it with you: it not only is the best way to get your key on subsequent occasions (the desk clerk may not speak English), but the card also will show your Metro stop, applicable bus routes, and will identify you as a tourist eligible to spend hard currency.

In the city, there are interesting book shops (Dom Knigi, which means the House of Books, has a big collection), department stores, and small antique shops, which the Russians call "commission shops."

Much of any tourist shopping in Leningrad will be confined to state-stocked stores called "Beriozka" shops (analagous to the Friendship Stores in China), found in the major hotels and in the center of the city. These are designed for tourists, and the merchandise, although priced in rubles, is sold for hard currency only, with the Russian ruble trading at about $1.40. (That is probably overpriced, because waiters and other adventuresome Russians eager to acquire dollars will be happy to trade you one ruble per dollar. But it isn't worth the potentially serious trouble you can get into if you're caught in black market operations.)

The best buys appeared to be high quality, but expensive, fur hats, other fur garments and liquor. Books are also well-printed and cheap, especially the guidebooks to museums. (The Hermitage Guide, with color plates, was 4.68 rubles, or about $7). There are also lots of dolls, toys and enameled boxes.

Color transparencies are also available, and are very cheap--although the quality is no better nor worse than mass-produced slides sold anywhere. Take your own camera and lots of film. There is no ban on taking pictures in the Hermitage or elsewhere, provided you don't use flash.

Some important DON'Ts:

DON'T drink the water. Leningrad water is infested with a particularly virulent amoeba which is difficult to get rid of. Don't even brush your teeth in Leningrad tap water. Don't drink water from pitchers on restaurant tables. Don't eat ice-cream (it is water-based) or use ice in drinks. Drink only bottled water, beer, Pepsi-Cola or other soft drinks.

DON'T forget to take whatever pills or vitamins you normally need: you're not in Paris or London.

DON'T cash too much money into rubles. Twenty dollars at a time is enough. Your shopping at the Beriozka stores will be done in hard currencies. Rubles are needed for postcards, stamps, Metro and taxi fares, bottled water, chocolate bars, etc. Water, beer and snacks usually are available at one or more "buffets" on the residential floors of the hotels.

DON'T carry along any obvious anti-Soviet literature or books/pictures the Soviet customs authorities might consider pornographic. It could be confiscated and cause a hassle.

DON'T expect that your passport will be stamped: you'll be given a visa that will be inserted into the passport, will be stamped, then taken when you exit the country.

DON'T expect price uniformity. If you see something you like, buy it, and don't worry when it shows up at a cheaper price. But check your change, and figure out the exchange rates.

DON'T hesitate to tell your guide what you want to see. Ballet and opera tickets are available, and very worthwhile. Push the guide to get the orders for tickets in early. The opera houses are magificent old theaters--themselves worth the price of admission. But you also get to see a Kirov-trained troupe.

If you're in a group, don't depend on anyone else to count heads: two of our party, concentrating on airport shopping, missed the plane in Helsinki. They arrived on the next plane.