It used to be true that 60 percent of all Puerto Ricans received food stamps from Washington. Not any more; now they get cash.

Puerto Rico again has been made an unwilling guinea pig for the mainland government, this time in testing the idea that a block grant of cash can provide the poor with food money more cheaply, more efficiently and more effectively than food stamps.

One month into the switch, the bureaucrats love it, but the recipients, who all get lower benefits, aren't so certain. In the meantime, Washington is again changing signals. Last week, the House Agriculture Committee voted to make Puerto Rico go back to some type of non-cash approach.

It is all rather frustrating for Puerto Rico's officials, who see this as another example of Washington's breezy indifference toward the commonwealth's fate.

"We didn't want to be special, but they went ahead and did it," said social services director Genaro Collazo. "Then they provided for us to develop our own cash system, and when we did they didn't like it. I don't really understand."

It also frustrates the working poor like Nidia Calderon, who said her monthly allotment went from $315 in food stamps to $245 in cash to feed a family of seven. Selling hot corn from a pushcart to the street idlers in the gritty, pastel slum called Llorens Torres, Calderon said she had considered joining her brother in New York, "but there's no work there either."

Nearly all the 1,450 families in Llorens Torres' three-story buildings get the new cash grants. "Maybe 80 percent of us are unemployed here," said Maria Martinez as she sat by the cart. As a pointed joke, a friend of hers called to some men sitting across the street, "Hey, come over here. Jobs! Work!" All four immediately got up and came over.

"Every time they change the programs, the checks are later and smaller," Martinez continued. She said she used to get $212 to feed her family of five, but this month received a check for $182. Since 80 percent of Puerto Rico's food is imported from the United States, prices are as high or higher here than they are in Florida. "I stopped buying meat," Martinez said.

The cash grant approach was pushed through Congress last year by Sen. Jesse Helms (R-N.C.) as a means of putting a roof on Puerto Rico's growing food stamp costs. The Reagan administration also saw it as a trial run for the controversial block grant approach to aid programs it would like to try nationwide.

The welfare debate has always been divided between those who favored the "cash-out" approach, arguing that adult recipients know best what their needs are and should be able to buy what they want, and those who want every dollar assigned to a specific purpose such as buying food in order to cut down spending on frivolous items.

This time, conservatives who normally favor strict accountability found themselves pushing the cash approach in order to save administrative costs such as those incurred in handling paper like food stamps. Liberals, on the other hand, worry that the block grants are an excuse to slash overall spending and a first step toward abandoning the entire program.

With half the per capita annual income of Mississippi and unemployment at a staggering 24 percent, Puerto Rico is by all standards poorer than any state, so it often has been treated as a special case in social welfare legislation.

The laws have limited what it can get for Supplemental Security Income, Medicaid, aid to families with dependent children (AFDC) and so on, with the result, according to the governor's office, that it gets less total federal money per capita than mainland citizens: $1,250 to $2,400. Such payments still total a third of the island's gross national product.

But food stamps were made freely available when Puerto Rico was finally given the program in 1974, six years after it was launched. By last year, nearly 10 percent of all federal food stamp spending, about $1.1 billion, was going to Puerto Rico.

"There was a massive black market" in food stamps, which could buy anything here, said a prominent economist who did not want his name used.

The stamps also became a political issue as officials debated, as they do on the mainland, whether the system was responsible for a decline in the number of unemployed people seeking work, for an increase in drug addiction, crime and suicide rates, or for a higher number of menial jobs that appeared to be going begging for lack of applicants.

Independence Party chief Ruben Berrios argued that food stamps were the reason he only got 6 percent of the vote in the 1980 elections. "They are like drugs . . . . We tell the peasant farmers that in a republic they would cultivate their own land and sell the produce on a protected market, but it's hard to convince them they wouldn't die of hunger without those damned stamps," he said.

Defenders argued that on the contrary, the stamps had saved the population from hunger and malnutrition during the recession that hit the island after the 1973-74 Arab oil embargo.

Last year, Congress voted to cut nutritional aid to the island by 25 percent, setting up an $825 million block grant within which, the law said, Puerto Rico could do pretty much as it liked.

What it did was cut 33,672 families off the food rolls, reducing the income ceiling to $8,000 a year and slashing the limit on personal assets from $1,500 to $1,000. Then it replaced the stamps and 108 expensive local distribution offices with high-security monthly checks sent out from a central computer in San Juan, saving $10 million in administrative costs that made up for some of the cut, Collazo said.

Since Congress had suggested during the debates that the island ought to import less food, $25 million was set aside for special projects: promotion of local agriculture, pesticide spraying and a job bank program.

Now, 60 percent of the island's 3.2 million people receive food checks. They must register for work and show proof of it to cash the checks. In the first month, 80 percent were cashed in food stores, 12 percent in banks and the rest elsewhere, Collazo said.

He discounted published reports that wholesale food sales dropped 15 percent during July, saying that the program was too young to have caused this. He also denied that many cuts were much higher than the 12 percent he said was average, saying bigger slashes cited to him must have resulted from changes in the recipients' income status during July.

"Every day I'm happier and more convinced the change is good," he continued. Blanca LaFontaine, Collazo's auxiliary secretary for the program, said several states have written to see how the program was working.

But the House Agriculture Committee had second thoughts about the "cash-out" approach and voted recently to limit the Puerto Rico experiment to one year. The measure could reach the floor next week.

Meanwhile, recipients know mostly that their aid is less now than it used to be, even though they can now spend it as they like.

Nerida Rivera, who runs a sewing machine at a small needlework factory in Rio Grande, south of San Juan, began getting food stamps six months ago when her husband was laid off from his job as a solderer. She gets $3.10 an hour, and the $168 she received for food in July didn't quite manage to feed the family of four. "It would be much better for him to go back to work, but we have no choice," she said.

Nidia Calderon said those who want to cut food aid would change their minds with a visit to Llorens Torres. "Let them come here and look at this," she said. "They will see what unemployment looks like."