A Vietnamese immigrant, despondent over his status and living conditions in the United States, fatally poisoned himself and his 6-year-old son and left his wife and a second son critically ill from the dosage in the family's New Carrollton apartment Saturday night.

Rescue workers called to the scene about 10:30 p.m. by the man's sister-in-law found Duc Vuong Pham in the bathroom and his wife and two children in a bedroom, all unconscious. The sister-in-law, Thuy Nguyen, one of two relatives who shared the family's two-bedroom apartment at 5436 85th Ave., said at least one of the victims had been in convulsions.

Prince George's County police said yesterday they had not determined whether the woman and children knew that what they were swallowing was poison. "It was administered to them," said Lt. Robert Miller. "It was extremely lethal and toxic and it took effect very quickly."

Miller said investigators found two suicide notes written by Duc Vuong Pham in the apartment, one in English, the other in Vietnamese. He declined to reveal their contents.

The family left saigon last year because of the Communist takeover and arrived in this county six months ago, according to Thuy Nguyen. The 34-year-old Duc Vuong Pham, who had been a wealthy pharmacist in Vietnam, had been working part time as a lab technician at Prince Georges Community College, where he also was taking classes in English.

"He was very depressed," said his sister-in-law. "In Vietnam he had a pharmacy and wealth. Here he couldn't be a pharmacist unless he studied for another five years. He felt he had to do everything again.

"He missed his family. They're still in Vietnam."

Hundreds of Asian refugee families have settled in low-income neighborhoods of Maryland's Washington suburbs, county officials said yesterday. Relatively few live in New Carrollton, where rents have climbed recently, in part because of the area's Metrorail service.

Thuy Nguyen, who has been in this country five years, said she heard one of the boys screaming about 10:30 p.m., a half hour after they were in bed.

"At first I thought maybe he had a dream," she said. "Then I heard them both screaming. Then my sister called us and she said the two boys didn't feel well. After that I ran in their room and my sister fainted."

Her brother-in-law was in the bathroom. "He breathed very hard in there," she said. "Then he stopped breathing."

He was taken to Prince George's General Hospital, where he died two hours later of cardiac arrest, according to a hospital spokesman. His 6-year-old son, Quoc Uy Pham died at Children's Hospital at 4:12 a.m.

The couple's 5-year-old son, Quoc Binh Pham, was listed in critical condition at Children's yesterday. His mother, Le Hoa Nguyen, 30, was in critical but stable condition at Doctors Hospital in Lanham.

Hospital spokesmen said physicians had not determined the type of poison the family swallowed. "All the blood tests have been drawn, but they won't be back for a couple of days," said a spokesman at Children's Hospital.

The spokesman said the 5-year-old boy was being assisted by a respirator. "He is very critical. He can't maintain his own temperature, and we're supporting all his vital signs."

Autopsies are scheduled for today for Duc Vuong Pham and his son.

Thuy Nguyen said her brother-in-law and his family had been planning to move to California in a few months. "I don't know why this happened," she said. "At dinner he didn't talk about troubles, just common things. I cannot believe it."