Fully one-thrid of the 535 members of Congress hold outside jobs, and only 18 lawmakers-- two in the Senate and 16 in the House-- make do entirely on the $57,500 a year they are paid as legislators.

And all members but those 18 supplement their congressional salaries by moonlighting as lawyers or officers of corporations and banks, by speechmaking and through outside investments.

Most members of Congress also have expanded their lucrative financial investments after coming to Washington.

The investments involve almost every area of business activity in the United States, and closely link the members of Congress with the very industries they were elected to oversee.

These are among the revealing findings of a study of financial disclosure statements by Congressional Quarterly, a weekly government affairs magazine. All members of Congress were required to file such statements this year for the first time under the 1978 Ethics in Government Act.

The statements show that the Senate, traditionally the "upper chamber of Congress," is the home of the most upper-income members. Although the disclosure forms are so vague that it is difficult to calculate precisely, at least 21 senators are millionaires. Another 10 may be millionaires. The House claims only 24 sure millionaires. It has seven maybes.

The richest member of Congress not surprisingly is a senator, H. John Heinz III (R-Pa) an heir to the H. J. Heinz Co. food products fortune. He listed his net worth as between $36 million and $50 million. Last year, he reported an unearned income from securities of between $437,101 and $836,000.

Sen. Edward M. Kennedy (D-Mass) reported an unearned income of between $288,401 and $581,500 from family trusts and businesses. He placed second among the top income earners in Congress last year.

But he apparently isn't the second richest member of the Senate. That distinction goes to Sen. John C. Danforth (R-Mo.), an heir to the Ralston Purina Co. family fortune. His net worth is somewhere between $5 million and $17 million.

The richest member of House is Rep. Fred Richmond (D-N.Y.), who owns $16 million in stock in Walco National Corp., a small machinery and electrical equipment manufacturer.

Congress' top moonlighter is also a senator, Sen. Herman E. Talmadge (D-Ga.), who is under investigation by the Senate Ethics Committee, reported holding eight outside jobs.

But his outside-income-producing activities didn't set him apart in the body. The average senator had $53,000 in unearned income from securities in 1978. And 27 senators received more than $23,000 in honoraria by giving speeches to various groups, many of whose members are lobbyists interested in legislation. Thirteen House members, most senior ranking congressmen with key committee assignments, received more than $23,000.

While the House Ways and Means health subcommittee was considering hospital cost containment legislation, its chairman, Rep. Dan Rostenkowski (D-Ill.), received $14,000 from health groups, Congressional Quarterly said.

The American Bankers Association led the list of honorarium givers last year as usual. It gave 16 members a total of $28,000. The Grocery Manufacturers of America, another lobbying group, came in second with $20,750. The Brookings Institution, a Washington-based think tank, finished a surprising third with payments totaling $16,900.

Congressional Quarterly also found that 54 senators and 105 House members face apparent conflicts between their committee assignments and their financial holdings.

Sen. Russell B. Long (D-La), for example, owns $1.2 million worth of oil and gas property, and had oil earnings of more than $100,000 last year. Long is chairman of the Senate Finance Committee, which writes key energy tax legislation. Talmadge, chairman of the Senate Agriculture Committee, owns a 1,388-acre farm.

Sen. Lloyd Bentsen (D-Tex.), another member of the Finance Committee, owns from $140,000 to $400,000 worth of oil companies' stocks and has more than $2 million in banking interests.

Sen. Rudy Boschwitz (R-Minn.), like several other senators, has sizable holdings in defense contracting firms, chemical firms, and mineral and timber companies that present a potential conflict on a wide variety of legislation that comes before the Senate.

Two senators kept their wives on their payrolls in 1978. Jeanette S. Williams, wife of Sen. Harrison Williams (D-N.J.), received $33,500, working for a committee her husband chairs. Patricia B. Young received $24,677 for being personal secretary to Sen. Milton R. Young (R-N.D.).

The only two senators who reported receiving no income outside their salaries were Paul S. Sarbanes (D-Md.) and Larry Pressler (R-S.D.).