The thrifts are about to be freed from shackles of interest-rate limit

United States banks and thrift institutions are about to be set free - or nearly so. After some 50 years of tight controls, they will soon be able to offer whatever interest rates they like on deposits of any size, with a few exceptions.

Recently, the Depository Institutions Deregulation Committee (DIDC), the federal agency set up to define and monitor the loosening of bank and thrift deposit rules, issued its final regulations on time deposits of 32 days or more. The new regulations are supposed to give these institutions more flexibility in managing their asset-liability structure.

For banks and S&L executives, it gives them the chance to show what imaginative and careful management can offer. Ever since the rise of the money market funds, these executives have said, in effect, ''If we could only offer competitive, market-based interest rates to our depositors, we could compete on a level playing field.''

For the public, the DIDC action increases the prospect of finding interest rates with a good ''real'' rate of return over inflation. Already, rates of 8.25 percent to 9.18 percent can be found on the money market deposit accounts (MMDAs) that became available, without interest rate ceilings, in January. A rate of 9 percent yields a real return of about 4 percentage points over inflation, currently under 5 percent.

The MMDAs, however, are not considered ''time deposits'' because money can be put in and taken out of them at any time.

Under the new rules, interest rate ceilings on time deposits of 32 days or more have also been eliminated. Starting the first of next month, the only deposits with interest rate ceilings will be ordinary NOW accounts, passbook savings accounts, and time deposits of 7 to 31 days. Interest rates on these instruments will range from 51/4 to 51/2 percent.

For the others, rates will be determined by the marketplace in the area and what a bank thinks it can afford. This means banks will be able to offer certificates of deposit with rates that are not necessarily ''pegged'' to any particular instrument like US Treasury bills.

Also, unlike the earlier certificates, there is no minimum deposit requirements on instruments of 32 days or more, and money can be added at any time without extending the maturity of the rest of the money in the account. So if you want to add to it, you won't have to wait until you have enough money to start a new account.

There will be, however, at least one important difference between these instruments and the MMDAs or money market funds: early withdrawal penalties. For deposits of 32 days to one year, loss of one month's simple interest. For deposits with original maturities or required notice periods of more than one year, the penalty is three months' simple interest. The penalty on deposits of 7 to 31 days, which is basically the loss of interest earned between the last deposit and the time of withdrawal, remains unchanged. In all these cases, the penalty applies only to the money being withdrawn. If it wants, a bank or S&L can impose stricter penalties of its own.

The early withdrawal penalties on 32-day-plus deposits should make it possible for banks and S&Ls to offer higher interest rates on these instruments than they do on MMDAs. If people are willing to tie up their money for over a month - or over a year, in some cases - they should expect to be rewarded for it with a higher yield.

All of these accounts will, of course, be insured by the federal government up to $100,000, so safety is not an issue here.

Thus liquidity, or the ability to get at your money any time without penalties, will be your only consideration. If you do open one of these new accounts, even though the deposit minumums are low, they should be thought of in the same way as 6- or 12-month certificates. Only put money in these instruments that won't be needed for a while.

If you think you are going to need the money, you may as well stick with a MMDA. Or you can stay with a money market mutual fund, where the minimum deposits are low (about $1,000), there are no early withdrawal penalties, and you can move some or all of your money to a stock or bond fund if that looks better at the time. Depositors come first

I have just been notified by my savings and loan that they are planning to convert from a federally chartered mutual association to a publicly held savings and loan. They state they will continue to be insured by the Federal Savings and Loan Insurance Corporation (FSLIC) to the legal limits and will continue to be a member of the federal Home Loan Bank system.

If the conversion is made, will my deposits be as safe as they are now? Are they required by law to insure funds with FSLIC? In the event the savings and loan should fail, would stockholders take precedence over depositors? -- F. C.

As long as your savings and loan is staying with FSLIC, you should have no worry about it ''going public.'' Many S&Ls are doing this these days, in an effort to increase their capital so they can expand, offer new services, or buy smaller S&Ls in new markets. But they are not giving up deposit insurance, so your money will be as safe as before. An S&L may not be a member of FSLIC, but most are, and those that are not are insured by similar agencies in their home states. In some cases these state agencies are stronger financially than FSLIC. Finally, it is depositors, not stockholders, who would come first in the event of failure.

If you would like a question considered for publication in this column, please send it to Moneywise, The Christian Science Monitor, One Norway Street, Boston, Mass. 02115. No personal replies can be given by mail or phone. References to investments are not an endorsement or recommendation by this newspaper.