Leading senators, faced in 1984 for the first time with a limit on what they can earn in speaking fees, generally collected the $21,780 they were allowed and donated excess fees -- if any -- to charity.

Embarrassed by reports that 15 senators had received more than $50,000 in speaking fees from private interest groups in 1982, the Senate voted, by a narrow margin, in 1983 to limit honoraria to 30 percent of a senator's salary -- now $72,600 -- starting in 1984.

For some senators, the change has meant a substantial drop in income. Sen. Robert J. Dole (R-Kan.), who collected the most in honoraria last year, earned $111,429.88 and gave all but $20,350 to charity, according to his annual financial disclosure released yesterday. In 1983, Dole earned $188,917 in speaking fees, kept $106,917 and gave $82,000 to charity.

Sen. Richard G. Lugar (R-Ind.) earned $81,850 in speaking fees in 1984 and contributed all but $21,777 to charity. In 1983, he reported speaking fees of $132,450, kept $129,065 and gave $3,385 to charity.

The financial disclosure reports show that at least 57 senators collected and kept fees at, or near, the maximum limit of $21,780.

Ten senators refused to accept honoraria, and the remainder collected smaller amounts, including John P. East (R-N.C.), who received $800, and William L. Armstrong (R-Colo.), who donated his $600 in honoraria to charity.

Sen. Claiborne Pell (D-R.I.) donated his $5,500 in speaking fees to charity.

Sen. John H. Chafee (R-R.I.) collected $45,942.04 in honoraria and donated $24,182 to charity. Sen. Orrin G. Hatch (R-Utah) kept $21,777.50 in speaking fees and gave nearly $40,000 in additional fees to charity. Sen. Donald W. Riegle Jr. (D-Mich.) also kept $21,780 and donated $22,120 to Michigan charities.

The financial disclosure forms show that most senators received outside income from a variety of sources, including businesses and real estate and securities investments.

Sen. William S. Cohen (R-Maine) received $24,701.65 and Sen. Gary Hart (D-Colo.) earned $40,953 from book projects, including their recently released novel "The Double Man." A Hart spokesman said Hart's payments included royalties on a previous book and an advance on another.

There are a number of millionaire senators, according to the forms. But the forms require reporting only in general categories, so it is difficult to determine a senator's net worth.

Among the richest members of the Senate is newly elected John D. (Jay) Rockefeller IV (D-W.Va.), who reported 1984 income of $100,935.37, including his $60,000 salary as governor, plus at least $300,000 from trusts from three banks. Rockefeller listed at least $4.1 million in assets.

Other millionaire senators include John Heinz (R-Pa.), John C. Danforth (R-Mo.), Edward M. Kennedy (D-Mass.) and Pell.

Sen. Frank R. Lautenberg (D-N.J.), a founder of Automatic Data Processing Corp., one of the nation's largest computer firms, listed assets of at least $3.7 million and income of at least $1.4 million.

Other millionaire senators, based on the minimum totals, included Armstrong, listing $2.6 million in assets and $251,500 in income; Russell B. Long (D-La.), $2.6 million in assets and $1.03 million in income; Lawton Chiles (D-Fla.), $1.3 million in assets and $227,441 in income; John Glenn (D-Ohio), $4.1 million in assets, $895,984 in income, and Howard M. Metzenbaum (D-Ohio), $932,511 in assets and $323,365 in income.