What are perhaps the two best teams in baseball - Baltimore and Milwaukee - had a showdown here Monday night. Of 50,000 seats, 20,000 were empty.

Those vacancies neither indict fans here, who are suitably irrational about the Brewers, nor impugn the Orioles' appeal.

Rather, those empty seats in the upper deck at County Stadium demonstrate how quickly and quixotically the face of baseball is changing in this first season when the long-term effects of the free agent era may be appearing.

Even the knowledgeable fan has a hard time digesting a season when all four defending division champs - four established minidynastics, actually - are all dying or dwindling at once.

It's easy to get left behind, as the fans have been here, selling out entire series last weekend and next against New York and Boston, while embracing the Brids tepidly.

Last season the same division produced the Bombers and Bosox, the cream of the game then. Now, the Brewers and Birds may, in just a few months, supplant them. That's a charming sort of bewilderment.

The game's insiders sense this unaccustomed tumult is not happenstance, but rather the new and unstable order of things.

"The pace at which teams are built and then crumble has been accelerated," Harry Dalton, Brewer general manger, said yesterday. "At most, you can only look three to four years ahead.

"Success has always been the most dangerous tranquilizer. It dulls your instincts in the market place and erodes your curage to take risks. The security of victory puts you to sleep.

"That's more true than ever. You're always the last one to realize that your own stars are moldy. That's happened to both the Yankees and Dodgers. They've aged suddenly. When the Yanks traded Mickey Rivers, they were announcing that they'd given up on this year."

In the Orioles and Brewers, we see two teams that believe they have understood the message of the times: win now.

"You can't worry as much about the future, because no one can tell what the landscape's going to look like in just one year," said Baltimore Manager Earl Weaver. "Look at the two pitchers who started our series: Stone against Slaton. Both were the two best free-agent pitchers available after last season. We were the two teams that bought 'em, and look who's on top."

If the Yankee owner, George Steinbrenner, proved the first theorem of free agenty - yes, you can buy a pennant - then the Birds and brewers are trying to establish two more savory corollaries.

"The Orioles are trying to show that you can do it the old-fashioned way - witht the farm system and depth and teamwork, but only few stars," Dalton said.

"And we're trying to do it with a mixture of old and new. We're spending on both the farm and free agents.

"The priorities of producing a champion have changed. The order of importance now is probably farm system first, then free agents and, finally, a lot further down the line, trades.

"The Orioles accidentally proved, when they got burned so badly on the Reggie Jackson-Ken Holtzman for Don Baylor-Mike Torrez trade, that you just can't put a big deal together the old way.

"You have to please every player and every player's agent. It's too difficult and can blow up in your face too easily.

"The net result is that the game is more stable than people feared it would be, but less stable than it was in the old days when powerful teams stockpiled talent and stayed on top for decades."

What has evolved is a kaleidoscopic gamblers' paradise where the action is so furious that often even thep layers don't understand it. The emergence of Baltimore, California, Montreal and Houston as first-place teams is not the result of pixie dust but the double-edged consequences of gold dust.

"Teams are being forced into immediacy deals that may be horrendous in the long haul - like California giving up so much youth for Rod Carew," said Dalton, the Angel general manger two years ago.

"Your hand is forced. You have to wheel and deal while the cards are hot," Dalton said. "For instance, we have delayed an operation on Larry Hisle until this winter, hoping he can come back for 30 to 40 games this year that may win us a pennant.

"Now next year, when he'll miss the beginning of the season while he's still recovering, maybe I'll rue that decision. It's something you'd never have done in the old days, but 'next year' has become a very misty word in baseball."

The hallmark of this era is its precipitousness. "If you don't move ahead, you fall back - fast," Dalton said. "Look at Kansas City. They held a pat hand, even though (manager) Whitey Herzog screamed about it and warned 'em. Now, they've been caught and passed."The Dodgers fell victim to that same sort of overconfidence, the same kind that undid the Yankees in the mid 1960s.

"Just as the Yankees were so sure of their talent pool in the late '50s that they refused to sign bonus babies, so the Dodgers thought their organization was so good that they could turn their nose up at free agents.

"It may take five years or more for corporate policies to bear fruit, but they caught up with both of those teams.

"If you're not going to build the ideal way - with a star at every position and a manager to keep 'em all happy, like the Yankees tried to do," Dalton said, "then maybe the next best way is like the Orioles. You build a team with no deadwood whatsoever and great morale."