MEXICO CITY -- Mexican officials are seeking to play down a $1.5 billion U.S. loan guarantee to finance oil exploration and drilling by U.S. firms in Mexico, a reflection of the country's historic sensitivities about its oil industry.

The accord has taken on particular importance here because foreign participation in the oil industry is barred by the constitution, which both adversaries and supporters of President Carlos Salinas de Gortari were quick to point out after Treasury Secretary Nicholas F. Brady announced it Tuesday at the U.S.-Mexican summit conference in Monterrey. Brady said that, in return for the loan, Mexico displayed willingness to have U.S. oil companies involved in exploring for and drilling new wells.

The reaction was swift and loud because the government-owned oil fields have been a marker of Mexican sovereignty since they were nationalized in 1936 -- largely from U.S. companies -- and suspicion is widespread that the U.S. firms would like to get a foot back in the door.

Salinas, aware that any impression of giving in on this point would entail political risks, declared on his return from Monterrey that the constitutional restrictions will not be lifted because "this is the Mexican people's will and my conviction as president of the republic." The country's oil industry will be excluded from upcoming negotiations on a free-trade accord, he added.

"One of the subjects that President Bush did not touch on was petroleum," Salinas said in a speech to Mexican businessmen. "But given the subject's importance, and sometimes contradictory information, I did touch on the subject of petroleum. I did so to reiterate ... that the Mexican nation retains control and ownership over it."

Francisco Rojas, head of Pemex, the government oil monopoly, followed up on Friday by calling in foreign reporters to declare that, contrary to the impression left by Brady's announcement, the Mexican oil industry long has worked with foreign firms. The new accord, he added, will make a difference in quantity but not in principle.

"We always have been open to participation by foreign firms that can help us remain at the top of the latest technology," he said.

The constitutional prohibition against foreign equity in the Mexican oil industry does not mean foreign companies cannot work under contract with Pemex, he explained. "What we cannot do is share the new resources that would be discovered," he said.

Rojas estimated that the number of U.S. firms now working on contracts with Pemex is in the dozens.

In addition to these, a number of companies from France, Japan and other countries are working on contracts with the Mexican industry, particularly in offshore seismic tests in the Campeche fields in the Gulf of Mexico, he said.

In response to questions, however, Rojas acknowledged that no U.S. firms are working now on contracts for exploration and drilling. U.S. companies have been absent from Mexican drilling projects since the oil boom of the 1970s, he said.

Aside from the perennial concern over sovereignty, the issue has been complicated here by the U.S. desire to see Mexican oil production expanded to relieve pressure on a world oil market squeezed by the Persian Gulf crisis and to guarantee nearby sources for the United States in the event of disruption in the Middle East.

Mexico exported 1.24 million barrels a day in the first nine months of 1990, sending 60 percent of total exports to the United States. The level reflected a decision last August to increase exports by 100,000 barrels a day in response to shortages created by the gulf crisis.

Since then, Pemex has increased its export level by another 50,000 barrels a day for a cumulative increase of 150,000 barrels a day since Iraq invaded Kuwait on Aug. 2, Rojas reported.

Should war break out in the gulf and oil become even more scarce, Mexico could increase its production level by only a fraction, he added. This is because the Mexican industry already is producing at near maximum capacity, he explained.

Rojas said there was no quid pro quo for the $1.5 billion Export-Import Bank loan guarantee announced in Monterrey. But the increased exploration and drilling by U.S. firms foreseen under the accord nevertheless seemed to coincide with U.S. desires for increased production potential.

U.S. officials said the $1.5 billion guarantee was the first step in an agreement under negotiation that could lead to $5.6 billion in Export-Import Bank guarantees over a five-year period.