They haven't turned up the music in the clubhouse. They haven't switched from beer to champagne.

The Atlanta Braves are acting as if a 9-0 record is an everyday occurrence. It is, instead, the longest season-opening winning streak in the 107-year history of the franchise. The Braves are one short of the longest in National League history after tonight's 5-3 victory over the Astros.

Brett Butler's two-run single and Bob Horner's RBI double keyed a five-run sixth inning that helped Atlanta to win No. 9. The Braves had been held to an infield single by Nolan Ryan (0-3) prior to the decisive inning.

The team that has finished in the bottom half of the National League West standings for the past eight years, the team that suddenly cannot lose for winning, has responded to its upward mobility as if it were planned.

That could be expected from the Yankees or the Dodgers. Not the young, downtrodden, impressionable Braves.

"The people in Atlanta and around the league are trying to romanticize this thing," said Joe Torre, the Braves' manager. "But the players are not getting carried away, not getting overly excited, and that's healthy. This is our attitude: 'Why shouldn't we be doing this? Why shouldn't we be winning? Why not us?' "

"The momentum is there, no doubt about it," said first baseman Chris Chambliss. "But everybody knows it is a long season. It's real easy in this game to get carried away with good streaks, and that's not happening here."

Relief pitcher Gene Garber said: "From the first day of spring training, I think the guys thought we were a winning club. I think the guys are just feeling we are playing the way we are supposed to play. Everybody feels he is a winner. That's why the enthusiasm is tempered. We're doing what we feel we should be doing."

But how? How have the Braves, a team picked to finish no better than fourth, gotten off to the best start in baseball? Especially with their only established starting pitcher, Phil Niekro, on the disabled list for the first time in his career.

"Everything," said Torre, "has gone right."

The hitting. The team last year scored three runs or fewer in 64 percent of its games, largely because of the nonproductivity of Bob Horner and Dale Murphy. But this year, Horner and Murphy have hit six home runs in the first eight games and rookie leadoff batter Butler has reached base 18 times.

The pitching. Without Niekro, the Braves' season-opening rotation of Rick Mahler, Bob Walk, Tommy Boggs and rookie Steve Bedrosian had a combined major league record of 39-61. But Mahler pitched consecutive shutouts (and was the winner tonight), Walk has allowed only one earned run, and Boggs four earned runs. The short relievers, Al Hrabosky, Garber and Rick Camp, have combined for four saves and one victory.

The defense. The Braves won one game when left fielder Murphy went high to take a home run away from Houston's Phil Garner, another when second baseman Glenn Hubbard turned an apparent two-run single into an inning-ending double play. "I'd have to say," said Torre, "defense has been the most important ingredient of all. Defense and disposition."

Houston's Don Sutton said, "What the Braves are doing is kind of scary. I hate for them to realize they're as good as they are."

But Cincinnati's Dave Concepcion is not impressed. "They look the same as in the past to me," he said. "It's a long season."