REAL INTEREST rates have been drifting higher in recent months for the reason that inflation has been drifting lower. Real interest, you understand, means the nominal interest rate minus inflation. Low inflation is very welcome, but, as seems to be especially true of economic matters, every rose has a thorn.

The relationship among interest rates, inflation and the tax code deserves careful attention. What counts, for investment and economic growth, is real after-tax interest. Real interest is a rather slippery concept because it is based on a guess regarding the course of inflation over the life of the loan. People's guesses differ. It is also true that people's--and businesses'--tax positions differ.

If a family's income is high enough to put it in the 50 percent income tax bracket, a 14 percent mortgage means an after-tax rate of only 7 percent. If that family thinks the long-term inflation rate in this country is going to be 5 percent, it will conclude that the real after-tax interest on its mortgage is around 2 percent--not so bad. But for the more typical family, in the 19 percent tax bracket, the same 14 percent mortgage means an after-tax rate of 11.3 percent. If those people expect long-term inflation to stay close to zero, as it has done for the past half-year, their real after-tax rate is over 11 percent--an intolerable burden.

Similarly, a profitable corporation using a moderately high inflation forecast may calculate that bonds at 11 percent work out to very little real interest cost at all. But an unprofitable corporation, with no taxable income from which to deduct interest payments, will find the same 11 percent bond rate to be prohibitively high--particularly if it thinks inflation will be low.

The perverse effect of expectations here compounds, incidentally, the dilemma of the Federal Reserve Board. People who expect inflation to rise will not find present interest rates too high for borrowing and investing. People who think that the Federal Reserve can succeed in holding inflation down will find present interest rates dauntingly high.

Beyond the purely economic implications, these differences raise sharp questions of equity. No president or Congress ever intended to provide lower effective interest rates to the rich than to the poor, or to the profitable corporations compared with the unprofitable. But that is the consequence of the tax deductions--and those disparities are not small.