It is the theory of many psychologists that we all are products of our environment, an idea with considerable merit in the cases of Don Nelson and Billy Cunningham, head coaches of the NBA Eastern Conference semifinalists: Milwaukee and Philadelphia.

Tuesday, less than an hour before the 76ers' 112-108 Game 2 victory, Nelson sat in the press room of the Milwaukee Arena, sipping coffee and chatting amiably with members of his staff. Outside the Philadelphia locker room, Cunningham sat alone, trying to glean some final piece of information from a set of press notes.

When he was approached by a well-wisher, Cunningham dismissed him with a curt, "This is a bad time, a very bad time."

There would appear to be few good times for Cunningham, who has not lost the intensity that characterized his 11-year playing career.

"That's just my nature. I don't remember any season, any game, that I wasn't up for," says Cunningham, whose team has a 2-0 lead in the best-of-seven series going into Game 3 tonight in Philadelphia. "I'm competitive in anything I do. I like to try to win, to achieve the ultimate that life offers. If you don't do that, especially in the NBA, you're in the wrong business."

Though not nearly as talented, Nelson was also an intense performer during a 14-year NBA career. Like Cunningham, Nelson is a competitive sort who would prefer to place a bet on a game of Ms. Pac-Man than the Kentucky Derby because it gives him more of an opportunity to control the outcome.

Nelson was named the NBA coach of the year after the 1982-83 season and is a contender for the award this season. Cunningham has yet to be so honored and most likely never will. The reason? The environment each man functions within. "You're virtually eliminated when you're considered to have one of the top two or three teams," said Nelson. "That doesn't mean that the job you did wasn't as good, but it isn't going to be as recognized as the other guys'."

There is no question that during the greater part of Cunningham's eight seasons in Philadelphia, the team has been part of the NBA's elite. Appearing in the league finals in 1980 and '82 and winning the title the following season, Cunningham has won 63 postseason games, second all-time only to Boston's Red Auerbach.

Yet that success has been a curse at times, adding to the pressure that has caused him more than once to consider leaving the profession. "Every year when I finish a season I have to evaluate whether or not I'll come back," Cunningham said. "I ask myself a million questions. I look at my children and they're growing up so fast.

"People don't realize how long our seasons are. Because of our success, we're playing over 100 games a year and I have to ask myself if I can still coach this team and will they respond to what I say after eight years. There's not much I can tell them that they haven't heard 100 times before."

This season, if the message hasn't changed, the volume has. "I've been lucky. For some reason, he doesn't direct it at me as much, but he's raising his voice a lot more," said guard Maurice Cheeks. "I got here his second year. I don't know what that first one was like, but as the years have gone by he's gotten more and more intense."

There are those who say Cunningham is especially driven during this postseason. Last April, the 76ers, then the defending champs, were embarrassingly eliminated in the first round of the playoffs by the New Jersey Nets, who won three games at the Spectrum. While there wasn't a repeat performance this year -- a 3-1 win over the Washington Bullets taking care of that -- it has been rumored that anything less than a berth in the championship series could cost Cunningham his job.

The man who wields that power, 76ers owner Harold Katz, denied it. "Every year, someone says that if we don't do this or do that I'm going to fire Billy, but I've never put a timetable or said to him, 'You have to win x amount of games.' That conversation has never come up."

On the other hand, there's no denying Katz' omnipresence. Never hesitant to speak his feelings, Katz could make things so uncomfortable that Cunningham would walk out of the job. The owner of one of Philadelphia's largest travel agencies as well as a former co-owner of a downtown hotel, Cunningham is among the few who could walk out on a $400,000-a-year job.

Nelson is another. "There's enough self-inflicted pressure that you don't need any additional pushing going on," said Nelson. Nelson almost walked away from the Bucks at the end of the 1983-84 season, frustrated because his team constantly fell short of Boston -- and Philadelphia -- in the playoffs.

Deciding to stay because, "I didn't want to stick another guy with a bad team," Nelson was as surprised as anyone by the Bucks' 59-win season. "Things turned out to be special. It was a once-in-a-lifetime experience."

Nelson's relationship with management also is special. A close friend of former Milwaukee owner Jim Fitzgerald, the coach is developing a similar relationship with Herbert Kohl, who recently bought the team.

"It's difficult to find someone in that position that you can be close to, who feels that you're doing a good job and working hard," said Nelson. "It's hard for me to fathom having it any other way. The job is difficult enough as it is. There are some very enjoyable times in coaching, that kind of trouble would eliminate them all."

After Tuesday's game, Nelson stood in front of a podium, quietly discussing that night's loss, while in the Philadelphia dressing room, there were no smiles or theatrics from Cunningham. A large cigar, lit silently, was his only concession to a job well done.

"After we won in '83 Harold asked if I was coming back and I told him I really needed some time to think about it. I go through it every year, but despite the questions I come back to it," Cunningham was saying. "What would make me leave? When I can't bring the intensity to the game that I have now, then I guess it will be time to quit."