WASHINGTON (AP) _ The USS White Plains is an aging but essential support ship that serves as a floating grocery store for vessels assigned to the Navy's 7th Fleet.

The 581-foot food stores ship has been based at the Pacific island of Guam since 1984 and was returning there Tuesday after a deployment to the northern Arabian Sea in support of Persian Gulf operations. It was attached to the aircraft carrier Constellation and its battle group during the deployment.

According to Navy records, the White Plains is more than 20 years old but there is no history of any past serious problems aboard the vessel involving accidents or its machinery.

The vessel, the fourth in the so-called Mars class of food supply ships, carries the Navy designation AFS-4 and was constructed by the National Steel & Shipbuilding Co. of San Diego, Calif. The ship's keel was laid down on Oct. 2, 1965; it was launched on July 26, 1966, and then commissioned into active service on Nov. 23, 1968.

It has since undergone several major overhauls.

The ship has spent its entire life plying the waters of the Pacific and Indian Oceans, at various points calling San Francisco and the Japanese ports of Sasebo and Yokosuka home in addition to Guam.

The White Plains joined the fleet in time for numerous and lengthy deployments during the Vietnam War and earned seven battle stars for its performance. Even after American troops were withdrawn from Vietnam, the White Plains drew additional duty in the South China Sea supporting warships that had been ordered to retrieve underwater mines sown in North Vietnamese waters.

The ship has participated in training exercises with a number of foreign navies, includes those of South Korea and Australia, and also served as a ''mercy ship'' following natural catastrophes.

For example, it carried relief supplies to Guam and other islands in the Mariana chain in 1976 after Typhoon Pamela. Later that same year the ship drew another special mission, ordered without notice to the Indian Ocean to support a carrier task force monitoring a flare-up in hostilities between Kenya and Uganda.