NEW YORK (AP) _ Investors in electric utility stocks have first-hand proof that you didn't need to be a high-rolling speculator to prosper in the 1991 stock market.

The shares of many power companies, historically among the most conservative of stock investments, were in a prime spot to benefit from changing financial conditions over the past year.

The Dow Jones industry-group index of electric utility stocks posted a 21.17 percent gain for the year.

That may look paltry beside a jump of 142.39 percent for the medical and biotechnology stocks, or the 111.16 percent rise of the securities-brokerage group. But it compared quite favorably with the 20.3 percent advance of the Dow Jones average of 30 industrials, the oldest and best known indicator of market trends.

''1991 was kind to utility investors,'' observed analysts at Dow Theory Forecasts, a Hammond, Ind., investment advisory service. ''The recession highlighted the group's defensive characteristics, and falling short-term interest rates pushed money into higher-yielding securities.''

Falling rates have long been a plus for utility stocks, mainly because lower rates increase the appeal of the companies' dividends among yield- conscious investors.

Of late, that effect appears to have been enhanced because yields on money- market investments have slumped to some of their lowest levels since the mid-1970s.

''The drop in short-term interest rates has greatly reduced the attractiveness of returns on money-market funds and certificates of deposit,'' observed Jack Lavery, research director at Merrill Lynch & Co.

''As a result, investors are looking elsewhere for investment opportunities. The drop in rates makes certain high-yielding electric utility stocks look more attractive.''

The recent enthusiasm for the stocks, however, has substantially lowered the yields available to new buyers. As of early this month, the Dow Jones average of 15 utilities was yielding 6.17 percent, down from 6.85 percent a year ago.

That compares with yields on utility bonds of 8.4 percent to 8.5 percent.

A primary reason why the stocks yield so much less than the bonds is shareholders' expectation of future dividend increases. Bond interest payments, by contrast, are fixed.

But just how much and how quickly an electric utility raises its dividend depends on a variety of uncertain factors, including the political climate at state regulatory agencies that act on utilities' rate requests.

''Electric utilities' recent performance has not been tied to utilities' long-term fundamental prospects, in our opinion,'' argued Sanford Cohen, analyst at Morgan Stanley & Co., in a recent report on the industry.

''In our opinion, the industry will find it increasingly difficult to compete with other investment instruments in 1992,'' he maintained. ''Investor concerns over utilities' regulatory exposure are likely to increase.

''We have already seen a sharp reduction in the level of allowed returns on equity granted by regulators in the second half of 1991 as interest rates declined.

''Also, a troubled economic recovery and rising tax burdens make it increasingly difficult for regulators to increase electric rates, resulting in 'skinnier' rate decisions.''

In addition, Cohen said, ''regulators are placing greater importance on public-policy issues.

''Historically, regulators have swung back and forth between supporting ratepayer interests or investors' pocketbooks.

''Now, state and federal utility regulators are increasingly using their rate-making authority to forward public-policy objectives - most notably pollution control, conservation and increased competition. This trend will in our view mean higher rates and lower returns.''

End Adv for Monday Jan. 13