William Parker, one of the three baritones who made it to the finals of the Kennedy Center/Rockefeller Foundation Competition for singers "for excellence in the performance of American music," made a successful and spectacular bid for the top prize, valued at $65,000, and won it handily.

When the name of the second-place winner was announced, automatically identifying the top man, Parker's mother, Louise Aprker of Butler, Pa., burst into ecstatic floods of tears. Although music never played a large part in the Parker household and a career in it seemed unlightly for a physician's son who was a Princeton language major, Mrs. Parker always enthusiastically supported her son's musical ambitions.

One of his strongest points Saturday night in the Center's Concert Hall was Parker's decision to sing an all-American program even though the contest rules permitted songs by non-american composers as well. In the allotted hour of singing, Parker offered music by Ned Rorem, John Jacob Niles, Lee Hoiby, Leonard Bernstein, Hal Lanier, Aaron Copland and Charles Ives.

It had to be another big plus for him that Parker projected every word of poems by Walt Whitman, Thomas Merton, Tennessee Williams, Lawrence Ferlinghetti and others into the Concert Hall with Whistle-clean clarity.

That the other two finalists, who, like Parker fought their way to the top out of an original field of more than 300 entrants, were also baritones is an unusual coincidence. Still more unusual is the fact that any similarity between the three ends with the word "baritone."

Parker's is a big, lyric baritone, resonant in the lower range, flexible on top, though not without a recurrent strain. He has the dramatic power to rub nerves raw with Ned Rorem's moving Whitman lines in "War Scenes." He can also easily swing with the Bernstein-Ferlinghetti "Penny-candy Store Beyond the El."

Parker, who is 36, is well-known in Washington, which has been his home for more than 10 years, since his days with the U.S. Army Chorus. He studied with John Bullock and has coached with Pierre Bernac. His pianist, whose contribution added immensely to the impact of Parker's singing, was William Huckaby. Parker, who has sung in all of Washington's major recital halls, has also won top prizes in Baltimore, New York, Marseilles and Paris.

His first place in the Kennedy Center contest gives him $10,000 cash, with a bonus of $5,000 a year from now on if he continues to emphasize American music in his concerts, together with a contract for a concert tour and recording contract.

The second prize of $5,000 went to Leslie Guinn, 44, who was baritone soloist of the National Presbyterian Church many years ago. The oldest of the semifinalists, Guinn has been chairman of the voice faculty of the University of Michigan for some years. He, too, is a solid lyric baritone, but he was no match for Parker, either in a repetition of the Rorem cycle or in richness of vocal and dramatic variety.

But watch out for the young man who took third prize. At 25, Sanford Sylvan was the youngest of the semifinalists. Both on Wednesday in the Terrace Theater and again on Saturday night, the judges were talking about his phenomenal feeling for song and about "what he has deep inside."

There were things no one else sang as beautifully as this young man from Boston with the lightest of lyric baritones, a voice the French call "bariton Martin" to describe its particular texture. All that Sanford sang had a rapture, a feeling for the beauty of words and music that is rare among singers. He needs only maturing and the natural deepening of his present gifts to be an extraordinary artist. There were ways in which he equaled or surpassed his rivals.

Judges for the finals were composer Alberto Ginastera, conductor Max Rudolf, Canadian tenor Leopold Simoneau and U.S. singers Jan De Gaetani, Beverly Beardslee, George Shirley and Donald Gramm.