As far back as he can remember, Thomas Lasswell longed to ride a raft down the Amazon River, trek through the equatorial jungle, visit native villagers and watch a piranha in action.

His wife Marcia thought the adventure "sounded like the pits."

"I hate mosquitos," she says. "I loathe rafting and I have absolutely no desire to look a fish in the face."

So in 1970 -- their 20th year of marriage -- the Lasswells took their first separate vacations. He rafted down the Amazon, she sunned herself in Mexico. They both loved it.

Since that time, the couple -- who have a marriage and family-therapy practice in California -- have each year taken a separate vacation, along with their yearly "together vacations."

"We see a lot of couples fighting about what to do on their vacation," says Marcia Lasswell, 53. "One gives in -- or they compromise -- and neither person has a good time. Nothing is worse than getting dragged along somewhere you hate -- unless it's being the person doing the dragging.

"As long as it's balanced with time off together, why shouldn't a couple have time alone to pursue those things they love? Any experience Tom has that makes him happier enriches our relationship."

Togetherness, adds Thomas Lasswell, 61, can be carried to an extreme.

"The truth is we love each other a lot," he says, "but we love each other even more when we get back.

"When you're with someone day in and day out you tend to forget what they look like. When I pick Marcia up at the airport after she's been away, she looks fantastic. I'm overwhelmed by the feelings I get when I see her. I wouldn't have that if we were together all the time."

Separate vacations came into vogue around the time Bob and Carol and Ted and Alice were exploring open marriage. While couples' counselors say open marriage are now "out," separate vacations seem to have stayed "in."

"It's an up-and-coming thing," says Lynne Rutan of the American Society of Travel Agents. "Today we're seeing lots of two-career couples who are used to being independent, who have more money and who constantly shuffle complicated schedules.

"I wouldn't say it's something every couple will be doing, but I'm hearing more and more about it from our members. There seems to be a growing feeling among couples that separate vacations may make for better relations during the rest of the year."

An increasing number of women want separate vacations, says Francine Hall, 37, a management consultant and business professor, "because our lives are so hectic we seldom get time to be alone.

"Women need to get off the treadmill and find out who they are besides a worker, a wife and a mother. Of course you want to have time with your husband, too, but a vacation shouldn't be a once-a-year thing. A long weekend alone sounds great."

Hall -- who co-authored The Two-Career Couple with her husband Douglas -- planned her first separate vacation this year, but canceled it because of a death in her family.

"I am at a place in my life," she says, "where it's important for me to have some time for myself. I spent years accommodating my husband's job, and I have two kids who are now taking over the house. The only place in the world I feel is mine is my office -- and even that I share with a graduate assistant.

"I just get sick of togetherness. To have some time to myself with peace and quite, no phone ringing, no one calling my name . . . it would be so rejuvenating."

"This human need "to stroke oneself," says Mary Ann Bartusis, 51, associate professor of psychiatry at the Medical College of Philadelphia, is a major benefit of separate vacations.

"It's particularly important for people who overextend themselves," she says, "to have time to nourish themselves without having to consider all the other people involved in their lives.

"People today -- women especially -- are juggling many roles. This gives you a chance to have just one ball in the air: yourself."

For those who "feel guilty about taking the time to vacation on their own," Dr. Bartusis counters: "Consider how refreshed you'll feel when you get back, and how much more you'll be able to give."

While some mental-health professionals share this enthusiasm for separate vacations, others express reservations.

"It may be a way of escaping from each other," says Helen Crump, a social worker who counsels couples at the Northwest Center for Community Mental Health in Reston. "Separate vacations can be a convenient excuse to avoid dealing with troublesome issues in the relationship."

The desire for separate vacations "may indicate a degree of alienation," says psychologist John Neulinger, founder of New York's Leisure Institute. He and his ex-wife, he notes, took separate vacations "only towards the end of the marriage.

"If you love someone you'd like to share the things you enjoy with them. Separate vacations remind me of people dancing without touching and going into a catatonic state two feet apart. I think something gets lost. It's a lot nicer to go into a catatonic state together."

Separate vacations "can be a brave adventure," says Michigan psychiatrist Peter A. Martin, "or an excuse to take advantage of the other person.

"If a couple is well-matched in values and goals and doesn't resent the absence of the other, then it's fine. But that's very rare. It's more common for one to want to go off alone and the other to be looking forward eagerly to spending time together. Then there's a power struggle that one person wins -- and winds up paying for."

A major drawback of separate vacations, contends Dr. Martin -- who edited a book on the relationship of leisure and mental health -- is that "vacations are a wonderful time for leisurely, pleasant sex. Which is not something you can do on a separate vacation, unless you engage in sex with another partner -- and that's a whole different ballgame."

The notion that "separate vacations means adultery," says New York psychotherapist Stephen Shapiro, co-author of Time Off: A Psychological Guide to Vacations, "is a popular misconception. Your partner may need reassurance that what you want is time on your own, not sex outside marriage."

"No one's marriage seemed to suffer," says Boston psychiatrist Carol Nadelson, 44, of a three-week trip to China she took recently with 25 women. "It was wonderfully fun," says Dr. Nadelson of her first separate vacation in 16 years of marriage.

"We get too fixed on what's 'normal' and assume that's what's right. The idea of separate vacations isn't even as new and trendy as it may seem. Men have been going off hunting together for years. Women take a week at a health spa and no one bats an eye.

"Like most marital issues," says Nadelson, who is professor of psychiatry at Tufts-New England Medical Center, "the question of whether or not to take separate vacations is a very individual thing.

"It depends on what the couple's values are, what they want from a vacation, how much time they spend together, how much time they spend together, how much time off they get and where they are in their lives.

"Certainly it can keep people distant, or be an excuse for not dealing with each other. But I think the healthiest relationships I've seen are those where the couple has enough mutual trust and respect to allow each other that independence."