A 16-year-old Southeast Washington youth was in critical condition at Children's Hospital last night after being bitten by a Gaboon viper, one of the world's most poisonous snakes, which had been stolen from the National Zoo.

The youth, Lewis Morton of 1307 Savannah St. SE, had been riding a Metrobus Monday night carrying a plastic garbage bag containing two of the vipers. According to a police officer who accompanied him to the hospital, Morton said he had found the snakes in the bag in Rock Creek Park.

Doctors said last night that although Morton was responding favorably to treatment, he would probably suffer permanent damage to the muscle and skin tissue around his right shoulder and arm.

Bus driver Jane White said Morton had boarded her L2 bus opposite the National Zoo and ridden to the end of the line at 15th and K streets NW, where he got off shortly before midnight Monday the garbage bag slung over his shoulder. "He grabbed his shoulder and came running back toward the bus," White said in an interview yesterday. "He leaned on a door saying, 'I've been bitten by a snake.' I didn't know whether to believe him or not."

Then she noticed two small, blood-stained punctures in the right shoulder of his jacket, she said, and called by radio for help. The snakes, meanwhile, began slithering out of the bag, which had been dropped on the sidewalk.

Within moments, a fire department ambulance and several police officers were on the scene, including one officer, Ray Harper, an experienced snake handler. Harper responded to the call, he said yesterday, "because I couldn't figure out what kind of snake was indigenous to 15th and K."

Harper and other authorities gathered up the two snakes, with Harper gripping one behind the neck and carrying it to the hospital for identification.

Morton, a seventh-grade student at Johnson Junior High School in Southeast, was put into an ambulance, complaining of cramps. He kept saying, 'I'm dying, please God, I'm dying,' " according to Frank Fishburne, a medical technician in the ambulance.

Morton was admitted around midnight and by yesterday afternoon was groggy but able to talk with nurses. His doctors said he was "responding to antivenin serum and . . . making progress," but remained in critical condition in the hospital's intensive care unit.

Doctors had worked through the night to treat Morton, summoning antivenin from several East Coast cities and zoos, and consulting by telephone with a doctor in Zimbabwe, where physicians are more familiar with treating such bites.

Police officer Harper returned the snakes, each nearly five feet long, to the National Zoo after they had been identified. An animal keeper, Michael Davenport, said, " 'Wow, those are two of the hottest snakes we've got,' " according to Harper.

Neighbors yesterday described Morton as a youth fascinated with reptiles who had at times kept a water moccasin and a copperhead, both poisonous snakes, in aquarium tanks in his family's apartment.

"His mother just refused to come inside until he got rid of the copperhead after it slipped out of its tank one day," said neighbor Robert Liles. "He just had a way with animals, though. He never seemed to get bit."

Zoo officials said yesterday that someone broke into the zoo's Reptile House sometime between 9 p.m. and 11 p.m. Monday. The break-in was not discovered until zoo officials got a call for antivenin from Children's Hospital officials who were treating Morton, sometime between midnight and 1:30 a.m. yesterday.

Just after 11:15 p.m., Morton boarded White's bus on Connecticut Avenue headed downtown, according to White. She said the youth was carrying a large tan trash bag and held it between his knees as he sat on the front passenger seat facing the driver.

"He didn't look nervous at all," White said. "There wasn't anything suspicious about his behavior." The bus was nearly empty when Morton got on, she said, and he was the only passenger on the bus when it got to the end of the line at McPherson Square. The youth left the bus on 15th Street and walked north toward K Street, she said. Then she saw him running toward her.

Morton said then that he had been bitten by a python, White said, and complained that he was seeing red.

Harper, who was in an undercover car patroling Connecticut Avenue, was among the first to respond to White's request for assistance. As a youngster, Harper said yesterday, he had caught rattlesnakes in Ohio and Georgia, and milked the snakes of their venom for money. When he arrived, he found one of the snakes sticking out of the discarded bag at the corner of 15th and K streets, he said.

Harper subdued the snake by pinning its head on the sidewalk with a pole he borrowed from a fire vehicle. He then picked up the snake with the pole, and grabbed it by the head.

"The secret is you have to grab them right behind the ears," Harper said. "But you've got to know where the ear is."

Harper, a 14-year police veteran, carried the snake into the ambulance, pinning it to the floor "so as not to suspend it from its neck." At the hospital, officials refused to allow him in, and he stood in the parking lot until Davenport arrived to identify the snake. Harper said that while he was waiting he obtained a cup from hospital personnel to milk venom from the snake "and the snake almost ate the cup."

Upon admittance, Morton's blood pressure was dropping and he was nauseated, lethargic and incoherent, hospital officials said.

The Gaboon viper's venom attacks the victim's nervous system, breaks down red blood cells, and exhausts the body's capacity to produce clotting agents in the blood, doctors said.

"I think we had very little time left at that point," said Dr. Muriel Wolf, who treated Morton.

Doctors administered Morton's first intravenous dose of antivenin serum from the National Zoo before 1:30 a.m. yesterday, hospital officials said. The zoo's stock of the serum, manufactured in South Africa, was enough for the critical first treatment, but Morton needed at least twice that amount to get safely through the night, officials said.

Hospital officials mounted a frantic search for more serum, and spent the early morning phoning zoo and hospital officials around the country looking for more. New York City police arranged an emergency flight from Kennedy Airport with serum from two zoos, while police used helicopters to fly in more serum from Baltimore.

Neighbors yesterday shook their heads in sympathy with Morton's plight. Said Robert Liles, "Maybe he'll leave snakes alone for a while.