Less than a day after suspending news briefings because things were going so well, the National Zoo announced yesterday that its panda cub died Friday night of respiratory arrest, apparently caused by a bacterial infection.

The cub's unexpected death dashed hopes that, after three pregnancies, this latest infant of Ling-Ling's would survive to be the first successfully bred giant panda in the United States.

As late as Friday afternoon, three days after the cub's birth, an upbeat zoo spokesman had said that the only reason for a future news conference would be to announce a name for the cub -- or "for bad news."

No cub name was handed out yesterday.

"We have heartbreaking news for you this morning," a somber, teary-eyed Lisa Stevens, the zoo's collection manager, told journalists. "Our giant panda cub died last night . . . . We're all very sad."

A zoo pathologist who examined the body of the cub, which was a female, said the infant panda incurred respiratory arrest because of fluid in its lungs and appeared to have peritonitis, an infection of the lining of the abdominal cavity. But he said he would not know the cause of death for several days pending further tests.

The death of the cub, which survived nearly four days, occurred just before midnight Friday, bedeviling zoo officials and keepers in their longtime quest to breed successfully a panda in the United States. And in a summer of the Iran-contra affair and presidential politicking, the sudden death of the baby panda seemed to matter a great deal to this government- obsessed capital.

Scores of curious parents and excited children headed straight for the zoo's Education Building yesterday morning, hoping to see the cub on a television monitor or at least view the videotape of its birth.

Instead, they were confronted with this bleak posted notice: "We regret to inform you that the panda cub died late Friday night. The cause of death is under investigation. The Panda House will remain closed throughout the weekend. There will be no showing of the videotapes."

Home town visitors to the zoo, and tourists who included it on their itinerary, were upset.

"I wanted to see the baby panda real bad," said 10-year-old Richard Hobson of Columbia, S.C., taking in the zoo with his parents. "I thought it would live -- but it didn't."

"It's disappointing," said Jennifer Saunders, 17, of Springfield, who was touring the zoo with her family and relatives from Texas.

Though clearly surprised by the cub's death, zoo officials said they will try again next spring to breed a cub -- if the aging Ling-Ling, 18, goes into heat.

"Every year we've learned something from our pandas," said Devra Kleiman, assistant director for research, who stressed the collaborative nature of the zoo's animal conservation efforts. She also chafed at the media's soap opera depiction of the zoo's panda couple, particularly an off-the-point question about how Hsing-Hsing, Ling-Ling's mate, was taking the loss.

She said the zoo is breeding the pandas not just because they are cute but also because "this is a highly endangered animal."

The population of giant pandas is declining. It is thought that only 800 to 1,000 survive in the wild. About 50 pandas are in captivity in China, and 17 are in zoos outside China. Six pandas have been successfully bred outside China.

Ling-Ling and Hsing-Hsing, gifts from the Chinese government in 1972, are the only panda couple in the United States, although another pair is on loan from China to the Bronx Zoo in New York. Ling-Ling is the oldest panda in captivity outside China and was pregnant twice before. Her first cub died of pneumonia in 1983 after living only three hours. A second cub was stillborn in 1984.

The third cub, born at 3:33 a.m. Tuesday, squealed loudly at birth and appeared to be healthy, zoo authorities said. Ling-Ling formed an immediate maternal bond with the infant, a relationship zoo officials said could have been jeopardized if they had handled the cub to examine it more closely.

"After 72 hours we thought we had it made," said Mike Connery, one of three panda keepers at the zoo and the attendant who was in the Panda House when the cub grew quiet and died.

Friday evening, Connery said, Ling-Ling had cradled the cub and panda watchers heard it "vocalizing" from the den, which was out of camera view. The last sounds from the cub were heard at 11:41 p.m.

"At 11:45 p.m., Ling went over with the baby to the water trough," said Connery, who observed the mother from near the enclosure. "At 11:56 p.m., she laid the baby down on its back on a pile of bamboo. The baby didn't move, and I figured something was wrong then."

At 12:05 a.m., according to collection manager Stevens, Ling-Ling picked up the cub and licked and cradled it. Again, there was no sound or sign of movement. At 1:43 a.m., the passageway to the panda's outdoor enclosure was opened and, after Ling-Ling went outside, the zoo's chief veterinarian, Mitchell Bush, removed the cub and found it dead.

Richard Montali, the zoo pathologist, said the five-ounce female cub was fully formed but had fluid in its lungs and abdominal cavity. He said death was likely caused by peritonitis but could also have been attributable to liver or circulatory failure.

"There was no evidence of crushing or mishandling by Ling," he said.

A male twin cub lived a few minutes before dying from a lack of oxygen. But Montali said he found no evidence of infection, leading zoo officials to believe that the surviving cub was infection-free.

After losing her two previous cubs, Ling-Ling cradled apples. Yesterday, however, zoo officials said that there was no such maternal activity and that the panda was alternately eating and sleeping.

The cub's death came as such a shock, Kleiman said, "because we thought we were over the hump." After one or two days, surviving cubs at other zoos have died only as a result of mishandling by inexperienced mothers, "so there was this feeling that we were pretty much out of the woods," she said.

Now, Montali said, the zoo will concentrate on what it has learned "each time we've had the good fortune of having a birth . . . . We hope next time to go further and raise a viable cub."