The Northrop Corp. has been blocked from selling its F5G fighter plane to the Taiwanese, but thanks to some friends in the White House, it may already have found another customer closer to home--the Defense Department.

Pentagon sources said yesterday a proposal is under study to buy F5Gs to simulate Soviet Migs in mock air battles against U.S. warplanes in training.

A Pentagon executive said White House officials had suggested that the Defense Department see if the Air Force could use the F5G, a faster and deadlier version of the F5E jet Northrop has sold in the past to Taiwan and other foreign countries. Northrop is based in California and the defense official said a number of Californians at the White House were interested in the fate of the plane; he indicated that the request that the Pentagon study the plane came through White House counselor Edwin Meese III.

Meese through a spokesman yesterday denied that he had asked the Pentagon to consider buying the F5G. But Meese, again through his spokesman, said he "admires the airplane."

So do the Mainland Chinese. So much so that they objected to its being sold by Northrop to neighboring Taiwan. The State Department confirmed yesterday that the administration will not allow the F5G to be sold to Taiwan. This puts even more pressure on Northrop to find a home for this plane it has spent--by its own estimate--$200 million to develop, mainly in hopes of sales abroad.

"This administration feels obligated to help Northrop with its F5G," claimed one Pentagon official who opposes going to the aid of Northrop. "That's why we're trying to find a way to use it even though the Air Force doesn't want it."

The Pentagon is expected to ask Congress to approve $100 million-plus in fiscal 1983 to buy new "aggressor" planes to fight against Air Force and Navy craft in mock dogfights in the sky. Older versions of the Air Force F5 and the Navy A4 simulate Migs in these aerial exercises today. Given its high-level backing, the F5G until recently looked to many in the Pentagon as a sure bet for the aggressor contract. But now Chairman John G. Tower (R-Tex.) of the Senate Armed Services Committee has become interested, altering those odds.

Tower is from Texas, home of General Dynamics, the nation's biggest defense contractor and builder of a slowed-down version of the F16. This slowed-down plane is built for export, just like the F5G. And it, too, could play aggressor.

"It has been suggested to me," Tower wrote Defense Secretary Caspar W. Weinberger last month, "that the Air Force may be proceeding with the directed procurement of follow-on aggressor aircraft beginning in fiscal year 1983. If that is the case, I would be grateful for your assurance that such an effort will not be initiated until all reasonable alternative aircraft which are suitable for the missions have been fairly evaluated."

A spokesman for Tower said yesterday that the senator did not write the letter to push the F16 but to serve notice that the committee would demand an accounting of how the Pentagon intended to modernize the aggressor squadron.

Thomas V. Jones, chairman and chief executive officer of Northrop, has known Reagan on a social basis. He attended, for example, the New Year's Eve party at Palm Springs, Calif., given by publisher Walter Annenberg for Reagan. Weinberger and Secretary of State Alexander M. Haig Jr. were among the Cabinet members at that party. A spokesman for the Northrop executive said he has no way of knowing whether business has been discussed between Jones and Reagan and other White House officials in social settings.

Jones in 1974 pleaded guilty to making illegal corporate campaign contributions to political candidates, including $150,000 to the 1972 reelection campaign of President Nixon. According to court records, Northrop Corp. money was funneled through a European consultant.

Within the aerospace industry and the Pentagon, Jones is considered a super-salesman for Northrop's products, shuttling back and forth across the nation and all over the world to see civilian defense executives, generals and admirals.

It would strengthen Northrop's sales pitch to foreign governments if the U.S. Air Force bought the F5G for itself. Les Daly, a Northrop spokesman, said his company is "not looking for special help" in selling the F5G abroad or to the Air Force as an aggressor plane.

In regard to the administration's blocking of F5G sales to Taiwan, Daly said: "All we want to know is what the policy is in regard to the 24-hour defense of Taiwan." As for the aggressor competition, Daly said the faster and more sophisticated F5G would be a logical successor to the F5s now used in those aerial exercises.

The F5G, thanks largely to its new 404 engine, is expected to have a top speed of about 1,300 miles an hour compared to about 1,000 for the F5E and would carry a bigger load of air-to-air missiles than the older version. The F5G could also be equipped for night as well as day combat.

Unless a buyer is found for the F5G, Northrop stands to lose millions on the plane it developed specifically for export under the FX program of President Carter. Daly, however, said Northrop already has written off most of its F5G losses. The first F5G is expected to be flown in September.