All day and all night, the heroin users stream in and out of the boarded-up rowhouse at 1321 Riggs Street NW, in groups of three and four. They laugh and chat as they walk inside, with the air of friends visiting a favorite bar.

Inside a shabby two room apartment on the second floor, they pay Linwood Thompson $1 each for the right to sit down and shoot up. Thompson, 31, a squatter in the apartment, is the proprietor of the "shooting gallery."

The District of Columbia is its owner.

"I know there's lots of drug activity in there," said Saul Finn, a director of the Department of Housing and Community Development. "We keep nailing boards up, and they [heroin users] keep pulling the boards down."

Finn said the rowhouse will be rehabilitated by a nonprofit agency within the next year and then sold. "We're just waiting for the City Council to approve a nonprofit sponsor" to rehabilitate the house, he said.

Homeowners along the block say they have complained repeatedly to the police and Department of Housing about the rowhouse junkies - with no results. "They don't deal up there," said one narcotics detective. "They go in with a tiny bag of dope. By the time they get in there, it's gone. They've shot it up."

Ten minutes later, they leave, usually looking no different than anyone else on the street.

They are bricklayers, cooks, government clerks, postal workers and the unemployed. Some are married, and many have children. Virtually all are in their 20s and 30s.

Thompson, who supports himself and his own $25-a-day heroin habit by operating the shooting gallery, figures he is rendering them a public service. "If they don't come here, they'll wind up doing it in a hallway or a car and get busted," he says, seated on his bedroom windowsill next to a vinyl couch, a soiled mattress and a large kitchen sink. The sink bears the sign, "The zink (sic) is for cleaning dishware only and not dirty tools or works (syringes)."

A tall, gangly man walks into Thompson's apartment and hands him $6 - $1 for the basic use of the gallery, $5 to have Thompson inject his arm for him. The man, a novice, has not yet learned to inject himself.

"You got a knife?" the man asks, as he struggles to open a tiny glassine bag. The room has an acrid smell, relieved only by a breeze that occasionally wafts through the window. "Where's the cooker," he asks, seconds later, as though preparing a cake. Thompson hands him an empty soda can.

The man stands next to the stove heating his heroin solution, and finally hands two full syringes to Thompson. Thompson puts one of them behind his ear, like a grocer with a pencil.

The two stand next to each other, in front of the sink. Thompson holds the needle on the man's arm and injects it, slowly. Then, he lets the syringe fill with blood and he injects it again. The man with the needle in his arm stands there silently, with his eyes closed.

Thompson takes the needle behind his ear and repeats the performance, inches below the spot where he injected the first needle. After several minutes, he removes the syringe.

The man, still standing there with his eyes closed, rubs his arm with his hand. Then he sits on Thompson's bed. His head is tilted downward, as though he is dozing. But he is stroking his face - again and again. His eyes are closed, but they flicker half open every few seconds, after his hand passes over his face. He looks as though he is very tired, yet cannot - or will not - fall asleep.

Water drips from the sink. A dog is barking outside. Finally, the man gets up and lumbers toward the door without saying a word.

Thompson is watching him. "He had too much," he remarks. "He lost control. See, he's holding the back of his pants."

Thompson, who was recently arrested and charged with possession of heroin, thinks of himself as a successful businessman. Like other businessmen, he likes a high turnover. Unlike most businessmen, he is not subtle about it.

He tells his customers that they have 10 minutes to shoot up and get out. "And if the police come," he says, pronouncing it po-leese, "don't throw it on my floors. Shoot it."

Thompson earns about $350 a week from running the shooting gallery. He has no overhead. He pays no rent. He has lived in the apartment and operated the shooting gallery for the past two years, ever since his former home, the notorious Whitelaw Hotel, was closed.

He says he found the Riggs Street apartment through a friend who used to live there, but moved out.

Thompson's customers estimate that about 100 shooting galleries exist in Northwest Washington - most of them in abandoned houses. They bring their business to Thompson because his apartment is ideally located, just a short walk from such outdoor heroin marketplaces as 13th and V, 13th and tw, 14th and T, 14th and S, and 14th and Swann.

Thompson tries to keep his shooting gallery competitive by keeping the floors neatly swept, by providing car seats and chairs, and by sterilizing the syringes that he distributes. "It would really bother me if someone caught something out of my place so I always boil my works (syringes)," he says.

Thompson charges an additional five to twenty dollars to inject men and women whose arm veins have collapsed from frequent punctures, or who have not yet learned to inject themselves. He injects toes for $5, necks for $10 and foreheads and backs for $20.

He does not charge to revive men and women who have overdosed on heroin. "That's on the house," he says. He has learned the symptoms: fingernails get pale white and then purplish, the mouth area becomes ashen, hands become cold and the person keeps asking for water.

"I massage their hearts and shoot salt water in their veins to bring them back. I'm not just making a dollar Sometimes I got to be a mother, a father, the police, the law, a doctor and a nurse. And sometimes your house got to be the hospital."

When heroin users can't walk home from his apartment "because of complications," - an event which happens about once each week he says - Thompson lets them stay for the night and feeds them lunch meat sandwiches.

Thompson has needle tracks on his arms. But he is alert and articulate. Unlike some heroin users, his eyes do not droop, and he conveys a sense of contentment and well-being.

He explains his appearance by drawing certain fine distinctions. "There's the heroin users and the heroin abusers," he says. "The heroin users know how much they can handle. The abusers just pack their bodies with it. They look older than you and me and my grandmother."

Doctors say Thompson's apparent alertness more likely stems from simply another kind of drug abuse: with his heroin he shoots up "speed."

Dr. Robert DuPont, former director of the National Institute of Drug Abuse, says it is possible - although not usual - for heroin users to maintain a low dosage over a period of years. "And in a shooting gallery," DuPont said, "the number not addicted is fairly small."

He said that 25 to 40 percent of heroin addicts are employed. "It's not as if they're all walking around in a stupor."

Thompson began using heroin when he was 16 and living in Southeast Washington. He stopped using it for two years when he says he studied at Georgia Tech, and also halted use while working as a clerk in a department store, and later as a lab technician for the D.C. Department of Public Health, checking blood samples for disease.

But when Thompson was 24, in 1972, he says, he was shot twice in the chest during an argument over a woman and was nearly killed.

He shows his gunshot scars - one above the heart and another in the stomach - with some pride. "I had 180 stitches," he says, pointing to a line of braid-like flesh above his navel.

He began using heroin again after he got out of the hospital. "When the doctor took me off medication, I felt pretty bad," he ways. "And I couldn't do much."

A young, pretty woman in an elegant, long being dress walks into the shooting gallery with two men, lightly linking arms with one of them. When the men aren't looking, she demurely pulls her pantyhose down and off, first from one foot, then from the other.

She is going to shoot heroin into her toes.

Minutes later, the three leave and another group of men and women pay Thompson and file into the back room amid falling wallpaper, scuffed wood floors and a window with a view of an alley. They have purchased their heroin several blocks away, several minutes ago.

Two men sit on car seats; another man sits on a chair, with a woman on his lap.

"It's just like going to the grocery store," says the woman, who identifies herself as a clerk for the Department of Recreation. "If you don't get a good price on one corner, you go to another one.

"Heroin is part of our culture," she says. "This is the environment that we have come up in.

The woman says she is a weekend user. Her male companion introduced her to heroin six months ago.

City narcotics detectives say "weekend junkies" do exist "but they don't stay weekend junkies for very long." according to one detective. "Pretty soon Wednesday and Thursday become part of the weekend."

Eddie, 25, is not a weekend junkie. He uses heroin every day. He packs his body with it. He can barely keep his eyes open. Sometimes, mid-sentence, he closes his eyes and stops talking. When he opens his eyes, he has forgotten what he was discussing.

He has been unemployed for two years now, ever since he got out of jail for possession of heroin. He lives with his parents and leaves their home every day around 1 p.m.

Then, he visits local supermarkets, stealing meat to support his $50 a day heroin habit. He says he sells the meat to friends and neighbors for half price.

"I walk out (of the supermarkets) real agressive, like it's mine," he says. "People don't question you if you are assertive."

In late afternoons or at night, he purchases heroin on on street corners, and then takes it to Thompson's shooting gallery to shoot up.

He says he uses heroin because "if you have a pain, a physical pain or a heartache, it takes away your pain. It's something you can forget for a few hours. It's a cure all for everything."

"But then reality slaps you in the face. Then it's time to get high again." CAPTION: Picture 1, Linwood Thompson sits in his heroin "shooting gallery" where customers pay $1 for 10 minutes to inject heroin. Thompson earns $350 a week in the business. By Linda Wheeler - The Washington Post; Picture 2, Linwood Thompson stands outside the city-owned house where he runs, as a squatter, a heroin "shooting gallery." By Linda Wheeler - The Washington Post