Ninety animal welfare activists from across the country, brandishing a 25-minute videotape that is to the animal rights movement what "The Silent Scream" is to anti-abortionists, staged a sit-in at the National Institutes of Health yesterday to protest the continued funding of brain-trauma experiments in a primate laboratory at the University of Pennsylvania.

The protesters, who had hoped to get arrested, settled down for the night in five NIH offices with overnight supplies of pita bread, humus, bean sprouts and tofu, after it appeared such arrests were not going to take place.

Earlier in the day, the demonstrators, ranging in age from 15 to 67, took over the eighth-floor offices of the National Institute for Neurological and Communicative Disorders and Strokes, chanting, "What do we want? Free the monkeys. When do we want it? Now!"

Alex Pacheco, chairman of People for the Ethical Treatment of Animals, which organized the protest, submitted a letter to Dr. Murray Goldstein, director of the institute, calling for an end to funding for "crude and unreliable experiments" on primate brains.

The protesters said they would not leave Goldstein's office, across the street from where President Reagan is recovering at Bethesda Naval Hospital, until their demands were met or they were arrested.

Protesters also demanded that NIH director Richard Wyngaarden agree to view a videotape culled from experimental records stolen 15 months ago by the radical Animal Liberation Front. The Front considers itself the guerrilla arm of the animal rights movement.

Over the 1984 Memorial Day holiday, Front members broke into the laboratory at the University of Pennsylvania, removing more than 60 videotaped hours of the experiments. The tape that PETA wants Wyngaarden to view is an edited version that protesters say shows the primates' reacting to pain and the researchers' callous treatment.

Later in the day, they modified their demand for an end to funding for the research, instead requesting a halt until an NIH review of the project, currently under way, is completed. Last night, William Raub, NIH department director for extra-mural research, said he was willing to meet this morning with the protesters, but that their demand for even a temporary halt to the experiments was "non-negotiable."

Pacheco, who said more than $13 million has been allocated for experiments on animal brains over the last 13 years, claimed animals at the Head Injury Clinical Research Center are not anesthetized.

"They use a screwdriver to pound the animal's head out of the helmet," Pacheco said. "They can't point to anything beneficial out of the experiments."

But Virgil Renzulli, director of the university's news bureau, said though animals "have to be awake" for some of the experiments, a "disassociative anesthetic" ensures that they feel no pain.

"What happens is because the monkeys may be moving their arms and legs, the animal rights people say they're not anesthetized," Renzulli said. "But that isn't true."

Among protesterss was Dr. Tom Regan of North Carolina State University, whose book, "The Case for Animal Rights," is considered one of the seminal philosophical tracts of the animal ethics movement. Regan said he expected Goldstein would "try to dismiss this as a demonstration by a bunch of animal rights crazies. But animal rights is on the map . . . and this is a legitimate case."

PETA staffers said the demonstration was triggered by the extension last month of an interim $500,000 grant to Dr. Thomas Gennarelli.

Gennarelli's laboratory at the University of Pennsylvania inflicts on the heads of baboons traumatic damage of the type often suffered by human accident victims. Researchers say that such experiments are necessary for the understanding and treatment of traumatic brain injuries.

According to Vicki Miller of the Washington PETA headquarters, "Wyngaarden had promised that NIH would investigate charges" that primates were left unanesthetized or subjected to unnecessary suffering at the Philadelphia center.