House Republican leaders joined yesterday in sponsoring a bill to help pay the costs of catastrophic illnesses for persons not otherwise covered by health insurance.

An individual would be required to pay initial costs approximateing 15 percent of income and the government assistance would pay most of the rest of longterm expensive treatment. A family with income of $16,000 a year would have to pay the first $2,400 before government help started.

The Senate Finance Committee is considering a catastrophic insurance plan drafted by Chairman Russell B. Long (D-La.) and Abraham A. Ribicoff (D-Conn.). House members handling health legislation are more interested in the comprehensive, full-coverage national health insurance sponsored by Sen. Edward M. Kennedy (D-Mass.), or the in-between program proposed by President Carter. But neither of these is yet given much chance to pass. Any program of national health insurance would be more likely to start with catastrophic coverage.

Republicans estimated their plan would cost $7 billion a year, compared to $17.6 billion for Carter's plan and $31 billion for Kennedy's.

The bill was revealed at a news conference called by Minority Leader John J. Rhodes (R-Ariz.) and endorsed by almost all House Republican Leaders and leading members of committees and subcommittees that would handle health insurance legislation. Rhodes said it was the first time he remembered that Republicans have got behind a health insurance bill in such an organized way.

The Senate voted, 78 to 0, yesterday to direct its Ethics Committee to make a comprehensive study of the Senate's code of official conduct with a view toward simplifying the rules of good behavior and report back in a year.

The proposal was made by Sen. Lowell Weicker (R-Conn.), in place of a resolution he authored last year to change the role of the Ethics Committee, repeal the limit on outside earned income of Senators and instead require more detailed financial disclosure.

His original proposal appeared certain to be rejected because both the Senate Rules and Governmental Affairs Committees had reported unfavorably on it. They felt it conflicted with the recently enacted Ethics in Government Act. They also said any proposed splitting of the now combined investigative and administrative functions of the Ethics Committee needed more study.

On Capitol Hill next week:

The House is scheduled to complete action on a bill authorizing $4 billion in water projects.

The Senate has scheduled votes on increasing military pay and extending for five years the federal aid program to help build airports and provide air traffic safety equipment. It may also take up the Federal Trade Commission authorization bill, which includes several new restraints on FTC regulation, and a patent procedures revision bill.