The Department of Agriculture's research center in Beltsville is carrying out experiments to produce super sheep and pigs -- perhaps twice as large as current livestock -- by injecting them with a growth-hormone gene from humans.

The researchers believe their work offers potential benefits in not only understanding how genes work but ultimately in providing a technique that will lead to a cheaper source of meat. Since giant mice bearing human genes have been bred using genetic engineering technology, the researchers said it is only a matter of time before they succeed.

But two scientific watchdog groups, calling the research a violation of "the moral and ethical canons of civilization," hope to keep the experiments from proceeding.

The Foundation on Economic Trends, headed by genetic engineering critic Jeremy Rifkin, and the Humane Society of the United States plan to file suit today in U.S. District Court here to halt the experiments. They charged in a statement that the tests represent a "new and insidious form of cruelty toward animals by robbing them of their unique genetic makeup."

In addition, they said, crossing the genetic material of two different species poses a potential environmental threat with unknown agricultural consequences.

"It is shocking that the U.S. government would condone experiments designed to place human genes into the germ line of other animals. I am sure that the American people will be disturbed to learn that their tax dollars are being spent on research that is a clear violation of the moral and ethical principles we share as a people," Rifkin said. A germ line consists of the genes transmitted from one generation to another.

The new suit will be a test case, Rifkin said in an interview, involving the fundamental question of whether it is permissible for scientists to use genetic technology to cross boundaries between species, since in nature unrelated animals generally cannot mate and produce offspring with mixed genes.

He has also asked a National Institutes of Health scientific board meeting later this month to vote on a resolution banning experiments in which genes from higher animals, including man, are transferred from one species into another.

Rifkin has already succeeded, in a novel suit against NIH that is currently under appeal, in halting government-funded experiments involving the deliberate release into the environment of genetically engineered organisms, such as microbes manipulated to help protect crops against frost.

Rifkin will be joined in the new suit by the Humane Society, a 200,000-member group that has long been in the forefront of the nation's animal-welfare movement.

"We have a Pandora's box. There is no assurance that new problems of disease and suffering will not arise as the genetic makeup of animals is altered," said Michael W. Fox, the society's scientific director. "We cannot improve upon nature until we learn to work with her."

But the researchers attempting to grow bigger livestock said they are proceeding cautiously and argued that the potential scientific and practical benefits far outweigh the theoretical problems raised by critics.

"My feeling is that if we could modify a species that will produce meat cheaper or faster for the benefit of people, I see no reason not to do it," said Dr. Harold Hawk, chief of the USDA's animal reproduction laboratory in Beltsville. While the genetic technology is new, man has long been breeding animals for his use, Hawk said, noting that farm animals have been so inbred that they are far different from their wild ancestors thousands of years ago.

While the human growth hormone gene is a good research tool and was used initially because it has been extensively studied, Hawk said, "We're planning before too long to switch to cattle genes."

He acknowledged, however, that the prospect of inserting human genes into farm animals could be unpalatable for many consumers. While he sees an individual human gene as "not really human anymore; it's just a chemical," he said that "too many people would be upset about that. These animals won't go to market for people to eat . . . . Some people would be disturbed about eating human genes."

The USDA is collaborating in the gene-tranfer experiments with a leading scientist in the field, Ralph L. Brinster of the University of Pennsylvania School of Veterinary Medicine, the first to demonstrate the possiblity of inserting a human gene into the genetic material of another animal.

In a study published last November in Science Magazine, Brinster and colleagues at the University of Washington reported that they had joined the human growth-hormone gene with a mouse gene and produced "transgenic" mice that not only produced the human hormone but grew up to twice as large as normal mice. They also showed that the gene became a permanent part of the genetic code of these mice and could be transmitted to subsequent generations.

Earlier, Brinster and colleagues demonstrated that a rat growth gene could be inserted into mice and affect growth.

In the new experiments, the USDA scientists provide Brinster with fertilized eggs from sheep and pigs at their Beltsville farms. He injects them with the human growth-hormone gene and the embryos are returned to Beltsville to be inserted into surrogate mother animals.

Hawk said they started with sheep about two years ago and later added pigs. Hundreds of embyros have been transferred: 30 or so at a time in one pig (pigs give birth to litters of up to 15) and five or so in a sheep, which produce only one or two lambs at a time.

Although most of the embryos do not survive, perhaps 50 of each species have been born so far. Both Hawk and Brinster said there has been no evidence of the animals growing larger than normal, the primary measure of success. But one source said there were preliminary hints that the human genes may be incorporated in some way in the genetic material of some of the pigs.

A complaint against the USDA by Rifkin's nonprofit foundation and the Humane Society will seek to have the gene-transfer experiments under the Agricultural Research Service declared unlawful as a federal common law nuisance and a violation of the National Environmental Policy Act, the Administrative Procedure Act and other statutes.

The Humane Society's Fox contends that the super animals could be more susceptible to disease, could increase the "factory farming" of animals in inhumane and crowded quarters and could change the scale of farming by putting small farmers out of business.

"By so profoundly modifying their genetic structure, we're reducing animals more and more to the level of biological machines . . . . Our position is that if you're going to exploit animals, we owe it to them to eliminate all avoidable suffering," Fox said.

"It's not cruelty to animals," USDA's Hawk said, adding that the experimental animals would be "closely monitored" and "certainly be tested in all sorts of ways" before they would move out of the laboratory.