The Post's editorial "The Saudi Planes" {May 22} misreads our basis for opposing the sale of F-15 fighter planes to Saudi Arabia. Our opposition to the sale pre-dates the tragic bombing of the USS Stark.

We oppose the sale because we believe that there is no convincing military or diplomatic rationale for it. Saudi Arabia has legitimate security concerns in the Gulf region, but these weapons would do little to address them. The F-15C/D models are designed for aerial warfare and are not equipped for ground-attack missions. They would be virtually useless to the Saudis in combatting the most likely Iranian assault -- a ground assault.

Since 1953, we have sold the Saudis more than $50 billion in weapons. The jury is still out on the benefits of these arms sales to the Saudis, who have consistently opposed the United States in the United Nations and been unwilling to join hands in the quest for peace in the Middle East. Before we sell them more arms, they should demonstrate stronger support for U.S. foreign policy and national security interests and contribute constructively to the peace process in the region.

We hope the administration will do more than just delay this proposal. Absent a change in the Saudis' distance from the peace effort, we hope the administration will abandon the arms sale altogether. FRANK R. LAUTENBERG U.S. Senator (D-N.J.) DENNIS DeCONCINI U.S. Senator (D-Ariz.) Washington

The Post's editorial on the Saudi reluctance to intercept the Iraqi jet that attacked the Stark overlooked some simple arithmetic. According to unofficial but reliable DoD and Israeli sources, the Saudis have ''lost'' four of the original 62 F-15s in their arsenal. If, as stated in the editorial, ''the new planes are replacements'' and the number of aircraft to be supplied are ''12 to 15,'' we are talking about at least a 20 percent increase in F-15 strength.

If the administration intended the planes to be one-for-one replacements, why has it not stated this categorically?

As for the limited fuel capacity on current Saudi F-15s alluded to in the editorial as an excuse why the Saudis were unable to perform the requested intercept, The Post should be aware of the previous sale to Saudi Arabia of conformal fuel tanks for their F-15s -- ironically, a modification developed by Israel.

MORRIS J. AMITAY Washington