In what may turn out to be one of the most important trips of his tenure, Secretary of Transportation Federico Pena leaves Saturday to pitch more than $6 billion in U.S. aircraft to Persian Gulf nations and to use air cargo as a potential crowbar to pry open Europe's aviation market.

Mr. Pena, whose four-day sojourn will take him to Saudi Arabia, the United Arab Emirates, Bahrain and finally to Belgium, will meet with top EC officials in an effort to promote a more liberal relationship through a multilateral negotiating framework.Currently, all U.S. aviation agreements, including those with the EC's 12 member states, are negotiated on a bilateral basis.

EC officials have been receptive in recent years to the idea of deregulating air cargo services with the United States, and Mr. Pena said this country's recently signed aviation accord with Germany, which formally deregulated cargo operations in the marketplace, could spur a move toward communitywide deregulation of air shipping.

"Cargo would be a natural place to start" toward achieving a broad liberalization package with the EC, Mr. Pena told reporters late Wednesday.

He stressed, however, that his discussions will encompass a wide array of transportation issues and won't focus on cargo deregulation.

Before Mr. Pena reaches Brussels, he will face the delicate task of convincing the Saudis to purchase 60 jets from Boeing Co. and McDonnell Douglas Corp. in a deal valued at $6.2 billion.

He also will meet with officials in the United Arab Emirates and Bahrain to discuss future aircraft purchases.

Last summer, supporters of Airbus Industrie said the four-nation European aircraft consortium had clinched the bulk of the Saudi deal.

In August, the U.S. Export-Import Bank agreed to loan guarantees to help finance Saudi Arabia's purchase of the U.S.-made planes. That news led lawmakers close to U.S. aerospace interests, such as Sen. Patty Murray, D- Wash., publicly to claim victory.

The pronouncements angered U.S. administration officials, who knew the deal was far from done.

''Those comments were premature and certainly not helpful," one official said.

Mr. Pena would not comment on the U.S. chances for success, saying any decision would come from the Saudi government. He did say, however, that a deal is unlikely to be announced during this trip.

In 1987, the European Commission, the EC's executive body, surprised U.S. officials with a detailed proposal to deregulate cargo services in the market and to put cargo issues on a faster track than the more controversial passenger operations.

The idea was quietly shelved, however, amid resistance from some countries regarding cargo deregulation and a general outcry by member states that the

commission was breaching community protocol by negotiating on their behalf.

But the issue has been kept alive by pro-deregulation forces in the United States and by European airlines that see cargo as an area where they remain competitive with U.S. carriers.

"We knew the Europeans felt more confident about beating us on the cargo front, and I think they still do," said Jeffrey N. Shane, who coordinated talks as a top policy official in the Reagan and Bush administrations.

Lufthansa German Airlines, Germany's leading carrier, controls about 70 percent of the cargo trade between the United States and Germany.

Its dominance is due largely to its reliance on five Boeing 747 jumbo freighters, which, with the exception of the huge Antonov A-124s operated by Aeroflot Russian Airlines, can carry more freight than any commercial airliner in the world.

In addition, German Cargo Services, a Lufthansa subsidiary, operates 10 freighters, three of them 747s.