(FORTUNE Magazine) – How to make executives behave in the best interest of shareholders? For years companies did it by subtly encouraging the brass to buy stock in their employer -- often by making shares available at a big discount. But now, subtlety is out. Companies are starting to make their honchos buy the stuff. Of 350 companies surveyed by management consulting firm Towers Perrin, 16% have adopted or are preparing ''guidelines'' that specify the amount of stock executives must own. Among them: Black & Decker, US West, and Union Pacific. Typically, under such a program executives are told they must buy -- over a period of five years -- an amount ranging from one to four times their annual base salaries. What if an executive fails to accumulate the ''suggested'' amount? Says Malcolm Kessinger, vice president of human resources at Union Carbide: ''There's no provision saying if you don't do this you will be terminated, but we have periodic reviews that cover how you're doing, and part of that is: 'How are you doing accumulating Carbide stock?' '' Other companies are even tougher. According to Towers Perrin, some say they would fire an exec who didn't buy sufficient amounts of stock. Other firms would cancel his bonus and stock option plan. It is too soon to tell whether the mandatory stock-buying schemes will boost returns to shareholders. One large institutional investor is surprisingly negative. ''We like to see executives in the same position as shareholders, but to force people to invest two or three times salary is undue strain,'' says a spokesman for a large institutional investor who did not want to be identified by name. ''You're changing the rules, and the executive is not going to do his job as well because you've disrupted his whole financial regimen.''