A WHITE FUR RUG in front of the fireplace. Two white sofas. He wouldn't have decorated in white when he was married, with a child in the house. A reminder of another way of life is poised at the doorway: a boy's toy truck. In this living room in a Georgetown townhouse, two worlds come together every Sunday, the worlds of the separated father and his son.#THe's the parent without custody. He no longer sees his child everyday, but maybe just on weekends. Along with emotional wrenching of all kinds, at first there's a singular fear centered on his child: that because he doesn't see him daily, he will lose his relationship with him.

He wants to make the now-telescoped time with him as enjoyable as possible. These are some ways he does it.

Nicholas Johnson, best known for his years as Federal Communications Commissioner, is a divorced father of three. When he and his former wife first separated in 1970, he would plan special activities for his children every weekend.

"You can get off into rountine where you're sort of entertaining your kids, taking them to movies, going out to dinner. That's okay, but they have to have a sense of how the father is living." Part of this compulsion to entertain he attributes to lack of imagination."It's a totally new situation. You're living in a small, bare apartment somewhere . . . It takes a long time to go 'round the barn and return to common sense." That is, to the place where a separated father begins to relax and do everydays things with his children.%TJohnson says the best times he's had with his two boys, 13 and 16, and his now married daughter, have been intimate conversations they've had, driving in the car back to their mother's house, working in the garden, sitting around a campfire at night - "When I have gotten to know them and they have opened up. I want to know them as people, and what we do is kind of background, like a wall you put pictures on - I'm interested in the pictures." He sees his children virtually every weekend.

Arno Zimmer is 32, a free-lance management consultant who lives in Alexandria, and the father of an eight-year-old boy. When he first spent weekends with his boy, he says, he would try to pack as many things as he could into two days. Movie, ice-skating, the zoo, Smithsonian . . . A familiar routine for some separated fathers. They've seen "Cosmic Awakening" at the Air and Space Museum three times. But now he tries to "normalize." If you spend all weekend doing things, he says, at the end of the time you may find yourself saying, "Okay, we've got 15 minutes left. What's on your mind?"

He spends every other weekend, Friday through Sunday, with Arnie, and what they do most together is read. Arnie's and avid reader of dinosaure books right now, and they've discovered some good books at the Smithsonian library. Other times, Zimmer likes do sports with Arnie, any sport, in a kind of personal crusade to expose him to all of them. Learning to skate, Zimmer says, "He came out on his ankles. I could see he was frustrated. I said, 'Arnie, get mad.' He did."

One Washington psychiatrist, Frank Kalibat, felt it was essential, as he puts it, to "maintain continutity," after his wife left, with their two daughters, six and eight. So for three years he continued to live in the family home, not even moving furniture around. At first, "I saw them every weekend go that they would know I had not abandoned them." They now spend every third weekend together.

INSTEAD OF finding things to do, Kalibat says, "I studiously saw to it that we deliberately did nothing. I didn't want to be a Disneyland daddy. What was most important was their relationship with me. I didn't want to dilute that by doing special things and giving them lots of presents."

Kalibat leans back in his off-white sofa - another person who doesn't have to concern himself so much with dirty hands and muddy footprints - and draws a deep drag from his cigarette. He lights one cigarette from another, smokes with enthusiasm, and talks about what he does with his daughters on weekends now, four years after the separation.

"We do things that we identify ourselves by. The carousel at Glen Echo. One fall we went to Sugar Loaf Mountain and at a roadside stand the kids picked out pumpkins. Last year we did it again, deliberately to retrace the steps. They know that this is something their family does, like ice-skating, horseback riding."

"But the main things we are centered here. They see this as their house, and they see the woman I live with as someone who lives in their house. What we do here is, we play, we chat, we listen to records. When 'The Nutcracker' was in town, we went to see it. The girls got a record of it. We remembered a couple of steps and we danced to the record."

ON HIS OWN, a father becomes more conscious of parenting. He may, of necessity, become more nurturing. Connecticut congressman Toby Moffett, who's 33 and divorced, spends summers here with his daughter, Julia, who's nine. Most of the day, she's in day camp. In the morning, "I wash or iron something, and make her lunch," he says. "I do a lot of sewing, washing and cooking."

He doesn't have a housekeeper. "Some people think I'm crazy, but I think it's important for her to see that I'm doing it all."

After she gets home from camp, she spends time at his office and might go onto the floor with him. Members' children can do that. He lives close enough to the Hill that he can go home, cook dinner and go back to the House floor for a vote when one comes up. On the weekends, they might have over a friend of hers, or go to the races at Rowie or go horseback riding: "Julia," he says with a grin, "is in training, she thinks, for equestrian events in Olympics."

Stanley Pottinger, a lawyer and former assistant attorney general, is a 38-year old father with custody of his three children, age nine, seven and four. He talks about what it's like to take care of them without a wife's help. "You find out what it is to hold two full-time jobs and how that is virtually impossible to do. You become incredibly more organized.

"Becoming responsible for kids you develop human traits - caring - that were typically reserved for half the population. I can remember being up to my elbows in dirty diapers during the Kent State grand jury, and wondering, How is it possible to do all this at once?"

WHAT HE DOES with his children is no different from what any father can do, whatever the living arrangements. For a birthday celebration, he rented a camper and took a bunch of kids to Kings Dominion. "I have found," he says, "they will go anyplace as long as they have a window over the cab.

"They love the zoo. I don't. They love to see 'To Fly.' But I don't think you have to leave home." They once built a tree house together. "A suspension tree house we saw in a magazine. I had just left the Justice Department, and it was the best thing I could have done for myself, as well as for them."

Pottinger suggests that a father living apart from his children may create problems if he mistakenly measures what he thinks of single fathers, and himself, against an unrealistic image of "the married father." More than that, "if they are not trapped by a sexits stereotype," he says, "they can make it."

SWITCHING FROM one parent ot another can be traumatic. Moffett's daughter cries when she has to say goodbye to her mother and cries again when she has to leave him.

"People talk about separation anxiety," Kalibat says."There's an anxiety of reunion, too. The first few hours are a frenzy. We have to be sure that we still exist, that we still know who one another is . . . Both kids compete for attention, jumping up and down, showing me things and telling me things and trying to be sure they get heard. Rivalry for my attention is an issue they rework every time."

It's especially difficult at first when the whole family's got to deal with feelings, along with changes in living arrangements, and still go to work or school, or - just survive.

"There's a human tendency to self-pity or guilt," says Aldo Beckman, The Chicago Tribune's Washington bureau chief and a father of five. "When I was first separated, I didn't have a car to use. Waiting at a bus stop with the kids, I thought everybody was looking at me thinking I was divorced because I was alone with my kids on a Sunday."

He sees his boy and four girls every weekend, and sometimes during the week, and he speaks lovingly of them: "I look forward to cooking for them on Saturday in my apartment. The ideal is if the kids could come over without ever packing a bag. Have, in effect, a small wardrobe. I expect I might do that."

SO MUCH OF having a good time with kids in serendipity, like one Saturday night Pottinger stayed home. The woman he dates (Gloria Steinem) was out of town. "Something wonderful happened," he says. "The TV broke." They spent the entire evening examining a globe that lights up to show the earth's mid-oceanic trenches and ridges. Sitting in the middle of the floor, they looked for exotic places, like Bora Bora and the street they live on.

Staging events for kids can sometimes make life difficult all the way around. First, children come to expect to be entertained, then to rely on it. And, as Kalibat says, "It makes for a not-too-subtle competition with the mother."

Moffett seems to have worked this out with his former wife: "She realizes the separated parent has a lot better chance to become Santa Claus. It's not the same and there's much more pressure when you see a child for a small amount of time."

Johnson, too, has confronted the question of competition: "Karen [his ex-wife] and I have tried to not make it the situation where the two boys are having a hilarious time with me and when they are with her they are doing normal, boring, workaday things."

EVENTUALLY, THE inevitable comes up: Can fathering and recoupling be combined? Dating is part of the weekend, too. And sooner or later there's a conflict, if only in the parent's head.

Kalibat says that for the first year, he avoided introducing women friends to his children. "It turned out," he says, "because there wasn't a woman around the kids were worrying about me. For them, grown-ups come in pairs.

Beckman had a similar experience. "At first, I was afraid they would resent it if I remarried. At first, I made an effort not to have friends there. But I think that was wrong and that is no problem at all now. They seem not to care." But kids will be kids. Once, alone with his six-year-old, he telephoned a woman friend. The boy recognized this and began to shout, "Mommy, mommy, when are we going to see mommy?" Kids are like that.

"My dating is way down in the summer," Moffett says. "I try to protect her from that." But when he does date, he finds "Women who have children make it more interesting for the kids. You're a step ahead of game."

Zimmer agrees. "If the other person has a child, that's fine. But that's very rare." It doesn't work for him to see a woman friend and his child at the same time. "I want to keep my priorities separated. They used to be understanding. I don't think he is. He complaints." He remembers "a disastrous brunch at the Montpelier restaurant.I spent about 150 bucks. Worst meal I ever had. My date left in tears, because my son was mean to her. Now I don't subject a date to that, and him neither."

IT'S UNFORTUNATE human tendency to take things and people for granted when you see them day after day, Johnson says: "The fact is, when I was at the Commission, and before that as Maritime Commissioner, I was into that workaholic program. There is something about living in a house where there are kids around all the time. It tends to make you say to yourself, and to them, 'We can't do it this weekend. I have to go to work. We can do it next weekend.' And it ends up you don't do it at all.

"Once you are separated, you are forced to plan time. They're coming in town for the weekend and you have to plan what to do. In a sense the kids take on more importance. They're not something that's there all the time and now you're forced to focus on them. Kids fade into the woodwork in a house."