Last spring, three of Finland's top industrialists put in an order for chaikas, the big, black Soviet limousines most often seen whipping in and out of the Kremlin. At a cost of $94,406 each, the purchases were considered unusual enough to make the Finnish papers. The point was not that chaikas were the best cars for the price, but that they were the highly visible symbols of a Finnish "buy Soviet" campaign.

Soviet-Finnish trade, long a key component in the delicate relations between the two neighbors, has reached the stage where it needs a boost. The simplest solution -- but not the easiest -- would be for Finland to import more Soviet goods and materials. Other options involve tinkering with the current trade arrangement.

Now, under a system of five-year barter arrangements dating to 1951, each side can buy from the other only as much it sells. The exchanges are made at prevailing world prices, and handled through accounts at the central banks in Moscow and Helsinki.

Under the barter arrangement, money never changes hands, so the Soviet Union neither gets nor spends its hard currency -- a commodity the Soviets guard carefully.

Finland and the Soviet Union maintain that the so-called clearinghouse system, a unique arrangement among the Soviet Union's western partners, has served both sides well.

The Finns say the Soviet market protected their exports at a time when other western economies were hurt by recession. For the Soviets, Finland has been both a guaranteed market for energy exports and a source of high-quality goods.

The Soviet Union accounted for 21 percent of Finnish foreign trade in 1984, equaling about $5 billion, making it Finland's top trading partner. Among western countries, Finland ranks second after West Germany in Soviet foreign trade.

While no one expects the basic system to change, there is talk now of rearranging it a little -- either by ushering in joint projects on a hard-currency basis or by trilateral trade arrangements, whereby a third country would become, in effect, the hard-currency clearinghouse for Soviet-Finnish trade.

"The history of Soviet-Finnish trade has been extraordinarily successful," said Pentti Kouri, a prominent Finnish economic consultant. "I don't think the arrangement is going to change, but I think it has reached the limits for the expansion of that kind of trade."

In recent years, the main problem in keeping the balance in Soviet-Finnish trade has been the price of oil.

Oil and other energy sources make up 85 percent of the Soviet Union's exports to Finland. When world oil prices fell in the early 1980s, so did Soviet exports and so did the ceiling on Finnish exports to the Soviet Union.

Since then, the issue for Finland has been what to buy from the Soviets to boost Finnish exports. "There is a saying in Finland that it is as hard to buy from the Soviets as it is to sell," said one Finnish government official.

Many items and commodities that the Finns want -- such as copper, scrap metal and nickel -- the Soviets do not want to sell. What the Soviets want the Finns to buy is machinery and equipment, which presently make up 5 percent of Soviet exports.

A small but visible part of this trade is the Lada automobile, the Soviet car modeled after the Italian Fiat. In Finland, the Lada -- particularly the cheapest model that costs about $4,600 -- is seen the same way as the Volkswagen Beetle was in its day in the United States.

Even the more expensive Lada models, costing almost $8,000, are a rare sight in Helsinki's middle-class neighborhoods. Some lay the resistance to Ladas to social snobbism mixed with latent anti-Sovietism.

Most, however, see it simply as an economic decision. The standard of living has improved steadily in recent years, and people want better, more modern and fancier cars -- not Ladas. Nevertheless, the Soviets have urged the Finns, sometimes not so subtly, to buy more Ladas. The Finns have found themselves explaining to the Soviets how capitalism works. "We have told them we can't sell their products for them. They have to do it," said one trade official.

But despite a sophisticated television advertising campaign and snazzy roadside billboards, sales of Ladas have remained relatively low. About 11,000 were sold in 1984, making the Soviet Union a distant fourth in automobile importers to Finland.

The Japanese, who offer the closest competitors in price to the Lada, were first in 1984 with almost 37.8 percent of the market, whereas the Soviets had 9 percent.

The Finns are looking at coproduction arrangements that can count under both imports and exports. For example, Tamro Corp., a large health-care firm, has signed an agreement with the Soviets to build ambulances that will be built on Soviet chassis.

Another major project in the offing would involve Finnish companies in a $1 billion project to exploit mineral reserves in the Kola Peninsula.