Senior Department of Defense officials, members of the U.S. military-industrial complex, Washington lobbyists and defense correspondents have their attention riveted today on the meeting of Spain's cabinet.

Word has leaked that this may be the day when a dozen or so men in Madrid finally choose either McDonnell Douglas' F18A or General Dynamics' F16 as Spain's "first-line fighter for the remainder of this century," as North Atlantic Treaty Organization strategists put it.

Money is involved. The order could gross either manufacturer $3 billion and sway similar decisions by Spain's Mediterranean allies, Turkey and Greece.

Prestige and politics also are involved. It is the Navy vs. the Air Force.

Navy Secretary John Lehman has written to the Spanish defense minister to lobby for the F18A, which the Navy is also buying. The Air Force, meanwhile, is rooting for its plane, the F16.

When money, prestige and politics are involved, controversy inevitably follows.

Last Thursday, The Wall Street Journal reported from Madrid that the Spanish Defense Commission had selected the McDonnell Douglas plane.

But a day later, the St. Louis Globe-Democrat, hometown paper of the competing aircraft companies, quoted Spanish sources as saying "no decision" had been made.

On Monday, Aerospace Daily, an industry newsletter, reported that the Spanish defense ministry had opted for the F18A pending receipt of answers about "offsets:" the concessions, such as coproduction arrangements, that Spain would receive in return for buying the planes.

At the same time, copies of Lehman's May 6 letter to Spanish Defense Minister Alberto Oliart suddenly began circulating widely.

One congressional aide described the letter as an attempt by the Navy secretary to "inject himself" into the Spanish decision-making process.

Several sources reported anonymously that Defense Secretary Caspar W. Weinberger and his deputy, Frank C. Carlucci, were angry about Lehman's foray into foreign policy.

In the letter, Lehman noted that repair and overhaul work on F18s with the U.S. Navy's Mediterranean fleet could be done at Spanish facilities "should such work be needed."

Lehman said late Wednesday that he had cleared the letter with Weinberger's office, and he insisted that he had made "no commitments" to the Spanish on use of repair facilities.

Other Pentagon sources acknowledged, however, that the Spanish decision is taking place against a backdrop of concern about rising costs of the F18 and doubts about the Navy's need for the F18A, the attack version of the plane.

When originally sought by the Navy in 1974, the F18 was to be its low-cost fighter and attack plane.

But the estimated cost per fighter has risen from $15.8 million to $28.8 million, and the estimated bill for the 1,377 F18s sought by the Navy is $39.7 billion.

The Navy claimed in March that most of the increased cost per plane has resulted from inflation. But some experts fear that the price of individual planes will escalate sharply in the procurement process, perhaps undermining President Reagan's budget plans.

Therefore, one option before the military planners is to buy more Grumman A6s as a main attack plane, instead of F18As.

Under this option, the non-attack version of the F18, which the Navy has just begun acquiring, would be kept for a fighter role.

However, an expected Spanish order for about 84 F18As would give a strong boost to the F18 program.

According to Lehman, the additional orders would enable McDonnell Douglas to produce the plane more cheaply and save the Navy $300,000 on each aircraft it buys.

In Canada and Australia, the F18 has been chosen over the F16. In both cases, the Navy gave a push. At the Navy's urging, the usual requirement that the foreign buyer pay some of the original research and development costs was partially waived, according to congressional sources.