Millionaire Sen. John W. Warner (R-Va.) picked up an extra $18,000 in income last year by selling two bulls, some cattle raised on his Middleburg farm and a painting titled "Fabrizia."

Sen. Harry F. Byrd Jr. (I-Va.), who boasts assets of more than $1.7 million, borrowed $37,000 to pay his income taxes.

Sen. Charles McC. Mathias (R-Md.) talked his way through the nation's chicken-dinner circuit to add more than $37,000 to his senatorial salary of $60,000. But Sen. Paul Sarbanes (D-Md.), despite his newfound notoriety as the target of the National Conservative Political Action Committee, could garner only $5,500 in speaking fees.

Financial disclosure forms filed by senators yesterday gave these glimpses into the private financial lives of the Washington area's four senators. Because there is no requirement to explain the information on the forms, they raise as many questions as they answer.

There was no explanation of why Byrd borrowed money, including $70,000 in 1980, to pay his income taxes. "That's a matter of personal finance," his press aide said yesterday. "The senator does not feel any obligation to go into it. He makes his disclosures and lets them speak for themselves."

A tax lawyer suggested that taxpayers with substantial holdings often can borrow at rates lower than the return they are getting on their investments, so it makes economic sense to leave investments intact. "He may be in a position to borrow right," the lawyer said.

What Byrd's disclosure form does show is that he maintains holdings worth more than a half million dollars in the Byrd family newspapers in Virginia, which includes The Winchester Evening Star and The Harrisonburg Daily News-Record, and owns stock in some of the nation's other major newspaper chains.

Byrd, who is retiring at the end of this year, purchased a condominium in San Francisco last year in which his daughter, Beverley, is living, according to a Byrd aide. The forms disclose only a range of value, so it could only be determined that the condo cost between $100,000 and $250,000.

Virginia's other millionaire senator, Warner, owes much of his wealth to the 2,100-acre Atoka Farm near Middleburg and Milldale Farm in Warren and Clarke counties. Each is listed on the form as being worth in excess of $250,000 -- the highest financial category. But in 1978, assessors valued Atoka at $1.7 million; Milldale at $1.1 million and Warner's house on S Street NW at nearly $800,000. Warner still owns the Georgetown house, but an aide said he lives in a two-bedroom cooperative at the Watergate.

Even Warner's fortune is not immune to the recession and other twists of fate, according to the form. The market value of a life-income trust given to him early in his first marriage, to heiress Catherine Mellon, has dwindled slightly. The trust, worth about $500,000 when Warner was elected, has dropped by about $50,000. "Municipal bonds, etc., have gone down," an aide noted. "The condition of the market, you know."

Also gone since his separation last December from Elizabeth Taylor are some perks that came with marriage to a superstar. Just before Christmas 1980, Warner traveled to London with Taylor on a six-day promotion tour for her movie, "The Mirror Crack'd." The trip, a gift from Sands Films Ltd., valued at approximately $3,000, was provided for in Taylor's movie contract.

Neither of Maryland's senators claim millionaire status, although Mathias lists assets of at least $265,000 for himself and another $210,000 for his wife. The liberal Republican is one of the Senate's most sought-after speakers. Last year he took advantage of a change in the restriction that had prohibited members of Congress from earning more than $25,000 a year for speeches. Congress voted to eliminate the ceiling and Mathias, who had reached the limit by October, made 13 more speeches for an added $12,000 before year's end.

Sarbanes, according to his statement, remained among the senators of most modest means. He listed as his only asset his Baltimore residence, which he valued at between $50,000 and $100,000. CAPTION: Picture 1, SEN JOHN WARNER. . . not immune to the recession.;Picture 2, SEN HARRY F. BYRDJR.,. . . borrowed $37,000 to pay taxes.