THE TALL, athletic-looking American was killing time in London's Heathrow Airport, waiting with his colleagues for a connecting flight. He glanced out a window and saw a plane bearing the insignia of Laker Airlines.

"Hmm," he mused."I had no idea the Lakers would charter a plane to come play in England."

Welcome to the European basketball tour. The tall American with the rather parochial view of the world is one of many marginal U.S. professionals who have flocked across the Atlantic to play ball.

For some there is good money to be had overseas, along with a measure of celebrity. Just as the North American Soccer League looks to non-North Americans to lure fans to U.S. stadiums, so too the European national leagues look to the Americans to help them fill their gyms.

There are about 600 Americans playing on European teams. Some are ex-pros and high draft picks making over $25,000 a year for top teams in France, Spain and Italy. Fewer than 100 fall into this category, and they include 56 Americans in Italy, half of whom are ex-U.S. pros and whose average salary is $40,000.

The majority of the Americans earn between $500 and $1,500 a month (usually with an apartment and a car thrown in) and are essentially cashing in on an opportunity to enjoy foreign countries while getting paid to do what they would probably be doing at home for nothing.

"I look at playing ball over here like going to school," said Kevin Goetz, former Boise State star and veteran of two seasons in Europe, one in Vienna and the other in Stockholm. "I get an education for eight months and then I can go back home and play soft ball all summer."

Jim McGuire, a '76 Yale grad, came to Oxford on a Rhodes scholarship. His scholarship has expired, but he will return next year for another go with an English team before eventual matriculation at Harvard Law School.

"I had thought my playing days were over when I left college," he said. "It seemed I was just giving up a gigantic part of my personality. My job here has given me an opportunity to keep playing at a reasonably competitive level and has provided for unique interaction with the English people."

The European market is open primarily to big Americans - centers and power forwards. Teams are limited to one or two foreign players and can fill the other spots with their own smaller countrymen. So American guards are scarce in Europe.

The level of competition varies over as wide a range as the pay. The lower division clubs in England, France, Switzerland, and Austria, for instance, would be outclassed by poor third division college teams here.

But the best teams in Italy and Spain could compete with any American college teams. They have Americans, usually, ex-pros and high draft picks, making as much as $50,000. Playing at this same high standard are the top Yugoslavian clubs, which rely exclusively on home grown talent.

Formal competition is strictly intranational, except for the various European "cup" competitions, which first division clubs can qualify for by finishing the season near the top of their leagues.

Cup competition, a foreign concept to the American sporting mind, is basically a single-elimination tournament taking place over the course of the European season, which runs from fall to spring.

The champions of each national league qualify for the following year's European Cup, the winner of which is declared European champion.

A cup match consists of home and away games, the winner being the team with the best victory margin, or aggregate score, from the two games.

The variety of competition levels - along with the great swings to pay - can make the American lifestyles abroad range between very laid-back and very tense.

With lots of lira or pesetas comes lots of pressure to perform.

Americans often soak up more than half of a team's budget. To justify their presence, they must produce. Before playing in Sweden and Austria, for example, Goetz signed a contract with a team in Belgium.

"They told me they wanted 30 points and 18 rebounds a game out of me," he recalled. "It was no problem for a while, but then my jumper was off one game and I only had 22. A couple of days later I lost my gig."

Most Americans playing on European squads have abundant free time. The Europeans, except for the very best players, have full time jobs on top of their basketabll obligations. Most teams have only one game and three or four practices a week. For the Americans, that means idle time. For some that means sight seeing.

Bob Nagle, a former all-conference center at North Dakota State, wastes no time establishing himself as a tour guide. On off days, he leads parades of tall people along the Champs Elysees and the Kuferstendamm, explores the Louvre and the Uffizi.

Once he organized an expedition into the meadows of Zermatt to see the Matterhorn. One of his teammates from California demurred from the trip. "Already been on the one at Disneyland," he explained.

Another American was similarly indifferent to France. "If you take away the sights," he observed, "Paris really isn't too impressive."

Some shrewd players have parlayed the twin benefits of European play - acculturation and good pay - into successful off-court ventures. Bob Perkheiser, ex-Purdue sharpshooter, plays for French powerhouse Villeurbanne and has married a French woman and established himself as a leading restaurateur in Lyon.

Fairfield alum Art Kenny played a number of years in Italy before becoming an executive in the American office of an Italian ship company.

Most Americans view their experience as a limited endeavor though, a break between college and an eventual career back in the States.

Joel Oberman has followed his four years at Valparaiso University with seasons in Helsinki, Vienna and Paris.

"I've had a great time over here," he said. "I've lived in three of the world's great cities. But it's time to go home and start making some money. The basketabll here isn't enough to sustain my interest, and I don't want to fall too far behind in the job market."

But others are less concerned about their return to the world of alarm clocks and two-week vacations.

Goetz, at 25, feels he could play for another four or five years. "I'd like to be able to tuck away enough dough so when I go home I can just play " golf for a couple of years," he said, "and not have to worry about plunging right into a job."