Bank and thrift regulators, in separate actions, took steps this week aimed toward protecting taxpayers from paying for financial institutions' risky investments.

The Office of Thrift Supervision, which regulates 2,389 savings and loans, proposed a rule that would require the stockholders of an institution gambling on interest-rate changes to put more of their own capital behind the institution's investments.S&Ls with more capital are better able to absorb losses and avoid failure, thus avoiding a bailout paid for by the government's deposit insurance program.

Since November 1989, S&Ls have been required to hold more capital against loans and other investments with a higher risk of default. A similar risk-based system takes effect for banks on Jan. 1.

However, Tuesday's proposal is the first by a financial regulator to link capital levels to the danger posed by interest-rate swings.

Meanwhile, the board of the Federal Deposit Insurance Corp. voted 5-0 on Tuesday to ask Congress for the power to tie deposit insurance premiums to risk.

Currently, all banks and S&Ls pay the same premium, no matter how risky their operations. Banks, starting Jan. 1, will pay 19.5 cents per $100 of deposits, up from 12 cents this year. S&Ls have been and will continue paying 23 cents.

Critics contend this unfairly forces sound institutions to subsidize high-fliers. Other kinds of insurance give breaks to better customers. Safe drivers, for instance, pay less for auto insurance and non-smokers pay less for life insurance.

A study prepared by the FDIC for Congress recommends exploration of two approaches for varying premiums. One would charge lower premiums to banks and S&Ls that hold a larger capital cushion. The theo-ry is that institutions willing to risk more of their owners' money before failure should pay less for their insurance, much as a homeowner willing to pay a larger deductible should pay less for theft insurance.

Another approach outlined by the study is having funds to directly cover 90 percent of a bank's deposits. It would purchase coverage from private firms for the remaining 10 percent and set the premium paid by the bank according to the best rate it could get from private firms.

The Treasury Department, which has been working closely with the FDIC, is likely to include one of the risk-based deposit insurance methods in a proposal it plans to release next month calling for the most dramatic overhaul of the banking system since the Depression.

Small banks, represented by the Independent Bankers Association of America, generally favor the insurance plan linked to capital, since most small banks have proportionately more capital than large banks. The smaller banks fear that private insurance would not be available to banks too small to be publicly rated.

The thrift office's proposal on risk-based capital would force banks to raise their capital by half of the amount it would lose in the event of a 2 percentage point swing in interest rates. For instance, an S&L that would lose $10 million from such a swing would have to hold $5 million more in capital than an S&L not subject to that risk.

Fixed-rate loans and bonds held by S&Ls generally lose value when interest rates rise. At the same time, the institutions have to pay more for their deposits.

"This is the latest in a series of steps taken . . . to see that thrifts safely manage their interest-rate risk," said thrift office director Timothy Ryan.

The agency noted that in the early 1980s, when interest rates soared, most S&L losses were caused by rising interest rates, not - as is the case now - by defaulting loans.

The agency is accepting written comments on the proposal for 60 days. It plans public hearings on Jan. 31 in Washington and Feb. 14 in San Francisco.