One of the key policy decisions that Corazon Aquino will have to make as president is what to do about Clark Air Base and Subic Bay Naval Base, two of the largest U.S. military installations abroad.

Aquino has said she does not want foreign bases on Philippine soil but would respect the current bases agreement until it expires in 1991. She has remained deliberately vague, however, about what she would do after the agreement expires, leaving room for maneuver on the issue.

The bases are important to Washington not only for communications, intelligence and projection of U.S. power in Southeast Asia and the Pacific, but also as a "back-door" supply route to the Persian Gulf and the Middle East. In recent years, they have taken on added importance, U.S. officials say, because of the increased Soviet naval presence at Cam Ranh Bay in Vietnam.

Although Pentagon officials say they intend to stay at the bases in the Philippines, they have begun to look for alternate sites, a relocation that could cost between $8 billion to $10 billion. Among the alternatives are Guam, 1,500 miles to the east, and Tinian, in the northern Marianas, where the United States has already leased 18,000 acres.

A Pentagon study on alternatives is due March 1. A report issued this week by the Congressional Research Service studies alternatives if Clark and Subic were lost. "But while agreeing that the functions performed by the Philippine bases can be relocated, these analysts also agree that the favorable environment and inexpensive work force of the Philippine bases cannot be easily duplicated elsewhere," the report said.

Aquino's advisers have interpreted her remarks about possible removal of the bases, if the regional situation permits, to mean that she would take such a step only if the Soviet Union were not in a position to exploit the move.

In an interview a week before the Feb. 7 election, Aquino reiterated that she wanted to keep her options open and that there would be negotiations on retention of the bases. "There is no head of state who has stated foreign bases should stay in that country forever," she said.

She also criticized what the bases, which have been in the Philippines for 83 years, have come to represent.

"We attribute Marcos' staying on to his connection to the bases," she said. "If these bases had not been here, maybe America would have withdrawn its support after he declared martial law [in 1972] and violated human rights so fully . . . .

Under the bases agreement, which has been amended numerous times since it was first signed in 1947, the United States has no legal obligation to provide compensation for the use of the bases, despite the interpretation during president Ferdinand Marcos' rule that U.S. military and economic aid was "rent" for the bases.

The United States does not describe U.S. aid as rent because it traditionally views the placement of bases in another country as in their mutual security interest.

The bases in the Philippines were justified for decades as outposts in a U.S. military arc for the containment of China. As relations with the Communist government in Peking improved, the rationale for the bases also changed.

After the fall of South Vietnam in 1975, Marcos became more vocal in demanding new arrangements. The Carter administration, after an initial debate over the value of the bases, agreed to renegotiation and in January 1979 the two sides signed an amendment allowing for five-year reviews.

In a separate letter to Marcos, Carter pledged that the executive branch would make its "best effort" to obtain $500 million in security assistance, directly linking aid and use of the bases for the first time.

In 1983, President Reagan pledged his "best efforts" to obtain $900 million in security assistance over the current five-year period, which began in October 1984.