During the past four years Mexico's Bolsa de Valores has become one of the hottest little stock exchanges in the world, full of fireworks and fiscal tremors -- a kind of financial Mount St. Helens.

Having lain dormant for most of its 86 years the stock market began to rumble promisingly in 1976 and in late 1977 it erupted spectacularly, blasting the index from 300 up to 1,800 points in less than a year and a half.

Although the big boom ended in May 1979, the bust has yet to come and the Bolsa continues smoldering at around 1,200 points, with financial action sensitive to every shift in the political and economic foundations of this country, so rich in new-found oil. The index is now watched as a kind of Richter scale tracing the mood of the nation.

Yet the market is still so immature and volatile that no one can feel confident making even general predictions about its performance.

Analysts basing their predictions on conventional patterns in other exchanges, and on the past record of this one, have been expecting another upsurge for months now. But it has not materialized and one devoted student of the market admitted recently, "We don't know what's going to happen. We've never had an adolescent Bolsa. We've only had an infant Bolsa." The bulls and bears in Mexico's financial district are still skittish calves and cubs.

Elements that give other exchanges at least a modicum of stability have yet to develop on the Bolsa.

Liquidity is a problem because there are no longer specialists capable, in effect, of wholesaling stocks.

This small market also has the potential for control and manipulation that does not exist on larger exchanges. It is estimated that there are only 50,000 investors, and fewer than 100 generally active stocks.

Although volume approached $14 million a day during 1979, it is only a third of that now.

"If a stock trades 10 million pesos a week," about $44,000, said one broker, "You have to consider that for the owner of a company, 10 million pesos is petty cash."

Another shortage at the Bolsa is of regular, widely disseminated financial information. Even the most sophiscated brokers must often depend on little more than hunches, tips and rumors to make their judgements.

"It's not an unsophisticated market," said one of th few foreign analysts working on the bolsa. "It's an ill-informed market. The only information, really, is inside information." No one can be certain who knows what and who is guessing.

As a result, there is much reading of omens and intense efforts are expended on divining the will of that nearly omnipotent entity in the Mexican pantheon of the powers that be, the government.

Although private enterprise is powerful in Mexico, there are some who argue that it, like almost everything else here, lives only off the tolerance of the state.

The government started the Bolsa bubbling toward the boom of the late 1970s when it passed the Securities Exchange Act of 1975, greatly increasing protection for investors, allowing for the expansion of brokerage houses and consolidating the Guadalajara and Monterrey exchanges with Mexico City's. An increase in taxes on short-term capital gains in undeveloped properties also helped lead investors into the Bolsa.

At the same time, however, the administration of then president Luis Echeverria appeared to be moving strongly to the left. When the peso was devalued drastically, the Bolsa continued drifting. It was not until the new administration of President Jose Lopez Portillo, the announcement of massive new petroleum finds and the recuperation from devaluation that the boom took place.

There were financial reasons for the end of the market's spectacular growth, but the trigger was once again a government decision. In May 1979, Lopez Portillo replaced three of the most influential ministers in his Cabinet. The index plunged along with them.

The Bolsa's sluggish performance this winter can be traced to the boom in precious metals and high international interest rates that made the market seem less than attractive. A series of electrical power cuts late in the spring did nothing to enhance investor confidence, but the business sector's nervers were especially frayed when Lopez Portillo visited Cuba last month. The shadow of socialism seem ominous to Mexico's conservative businessmen.

At the moment, beneath the modernistic vaulted ceiling of the Bolsa one can see a few brokers wandering rather listlessly about the floor, calling out their willingness to sell or buy while most bide their time.

The Mexican president's state of the union address is coming up Sept. 1, and no one had forgotten that Echeverria's announcement of the 45 percent devaluation came on the same occasion four years ago -- even if there is little reason to think anything so drastic is in store this year.

There is a persistent fatalism among Mexican businessmen, bred into them by the nation's tumultuous history. They seem to feel that anything good cannot last. But there is also intense opportunism, and if Lopez Portillo's message this year ultimately comes as a propitious omen, the Bolsa could easily be back on its way to a boom.