Defending champion Nancy Lopez heads a field of more than 60 players in the $75,000 Greater Baltimore Golf Classic that begins today at the Pine Ridge course in Lutherville, Md.

Only four of the current top 10 money-winners on the Ladies Professional Golf Association circuit will tee off in the charity tournament, the proceeds of which go to Baltimore's Children's Hospital and the John F. Kennedy Institute.

Lopez, who promised to defend each of the nine titles she won last year, is the tour's leading money-winner with $139,000. Others in the top 10 who are playing include Pat Bradley (No. 4) with $94,359, Sally Little (No. 9) with $64,530 and Hollis Stacy (No. 10) with $57,470.

The field is smaller this year than usual, reportedly for financial and scheduling reasons. Although the purse is $10,000 greater than last year's, the Baltimore tournament is one of three on the LPGA circuit that offers less than $100,000.

Unless the purse is raised, some observers say, the Baltimore tourney may be dropped.

This year's tournament also falls between the U.S. Women's Open and the Peter Jackson Classic in Montreal, one of the larger events on the circuit, so many players have skipped the Pine Ridge event.

However, Baltimorean Carol Mann, a perennial favorite, will be on hand, as will Kathy Whitworth and Marlene Hagge, Jerilyn Britz, who won the Open title last week, is not entered.

Mann has played sparingly this year because of television commitments and her golf schools. Whitworth has won 80 tournaments, only to Mickey Wright's 82 wins. Whitworth's career earnings total $846,120, the highest in LPGA history.

Hagge, who joined the LPGA tour as a teen-ager in 1951, has won 25 tournaments and earned $350,000. CAPTION: Picture, Nancy Lopez