The Commonwealth of Puerto Rico last week launched an experiment in the Reagan administration's brand of "new federalism." In the face of a 25 percent cut in federal food stamps aid-- previously the source of almost 10 percent of the island's personal income--Gov. Carlos Romero Barcelo decided to dispense with the stamp program entirely and mail out checks for cash instead.

Nutritionists and others concerned about the island's welfare are worried that, instead of buying food, people will fritter the money away on alcohol, gambling or nonessential consumer goods. U.S. Agriculture Department officials, however, doubt that much money will be wasted.

Changes in food stamp program rules in 1979 required participants to earmark less of their own money for food than they did previously. Since most participants in Puerto Rico are very poor, it's not suprising that they chose to spend less of their own money on food than the Agriculture Department thought they should and more on other necessities.

As a result, the Agriculture Department estimates that each dollar now given out in food stamps in Puerto Rico results in only about 15 cents more being spent on food than if people got cash. Gov. Romero figures that with program money scarce, it's not worth spending all that it takes to handle coupon issuance and redemption for a program that has so little proven effect on nutrition.

A more fundamental objection to the governor's program comes from critics who argue that the entire U.S. approach to helping Puerto Rico has been misguided, if well-meant. When food stamps came to Puerto Rico in 1975, the flood of stamps pushed up food prices and imports and may have hastened the decline of local agriculture. Substituting cash for food stamps may lessen these effects. The governor also plans to spend a small part of the program money for projects to stimulate local agriculture. But if most of the money is handed out to people who spend it on imported consumer goods instead, it won't do anything to help Puerto Rico's economic growth or reduce the dependency of its people.

The history of the Puerto Rican food stamp program has been a sobering lesson in the difficulties of transplanting home-grown remedies to different soils. But there is also a lesson for stateside programs in the new Puerto Rican experiment. If you give local governments the flexibility to spend money the way they want to, the results may not be just what you wanted. And if you give people free rein to spend money the way they want to, some of the results may be even less pleasing. But the costs of making people do exactly what you want may be unacceptably high.