When All-Savers certificates became available last year, some brokerage houses and mutual fund sponsors made arrangements to offer the certificates to their clients.

The certificates were generally provided with no sales charge or commission, both as a service to their customers and to permit the financial organizations to keep their fingers on the funds (which they knew would become available 12 months later).

A spokesman for one of the large fund families told me that 65 percent of the All-Savers certificates they had sold -- $72 million out of more than $110 million -- had matured and been redeemed through mid-October.

And she said that 78 percent of the maturing cash has gone directly into the family's taxable money market fund -- a statistic that surprised me.

I'm not surprised that most of that money is remaining with the managers of this family; it only seems to prove that their reading of the probabilities last year was valid.

But I am surprised that the funds have gone back to an investment that generates income subject to federal tax. I thought that, having been introduced to the advantages of tax-exempt income, most of these investors would now stay in the tax-exempt arena.

Maybe if I read my own writings I wouldn't have been so surprised. Just a year ago (on Oct. 31, 1981), I said that in my opinion most of the money going into All-Savers would be coming from relatively unsophisticated investors, not from those who were already familiar with the world of tax-exempts.

The data from a single mutual fund sponsor certainly doesn't represent a valid statistical sample from which to extrapolate. But it is an indication that perhaps I was right, and the bulk of the money coming out of All-Savers is now going back to passbook accounts, certificates of deposit and "standard" money market funds.

On the chance that that's what is happening, let me suggest to you who have maturing All-Savers to take advantage of this opportunity to take a better look at the investment world.

If the tax-exempt feature of the All-Savers certificates made sense last year, at least do a little research on other tax-exempt securities. That includes purchase of individual municipal bonds, the diversified portfolios of tax-exempt bond funds and unit trusts and tax-exempt money market funds.

Of course, it's a lot easier to just dump the money back into the old familiar places. But if getting tax-free money was attractive to you last October, then you owe it to yourself to take a look at some of the alternatives.

Since Feb. 1 the interest charged by the Internal Revenue Service on tax underpayments has been 20 percent. (The IRS paid the same rate on overpayments that qualified for interest.)

Now the IRS has announced that the interest rate will drop to 16 percent on Jan. 1, 1983, in accordance with new rules established by the Tax Equity and Fiscal Responsibility Act of 1982.

Under the new law interest rates are determined twice a year. The next rate change will be set by April 15 to be effective July 1, 1983. It will be based on the average prime rate for the six-month period ending March 31.

Preview of a new form: The IRS has added a new basic tax form to the 1040/1040A family, to be called 1040EZ.

Available for the first time for the 1982 tax season, the new form may be used only by a single taxpayer who claims no dependents, doesn't claim an extra exemption for age or blindness, and has taxable income less than $50,000, all of which comes from wages and less than $400 in interest income (but with no interest from All-Savers certificates).

My reaction is a purely personal one: Form 1040EZ may simplify return preparation for some taxpayers, but it will only add another complication to the preparation of The Washington Post Annual Tax Guide, tentatively scheduled for publication around mid-February of next year.