hroughout baseball history, pitchers and hitters have adjusted daily to the strike zone idiosyncrasies of the home plate umpire. Now umpires are adjusting, too, depending on the site of the game, and they say their adjustments can influence the outcome of games.

The reason, four umpires said yesterday, is the QuesTec system that Major League Baseball uses to monitor umpires' calls of balls and strikes and to rate the umpires on the accuracy of their calls.

The system is used in at least nine parks, including both in New York, and is expected to be in 13 parks as the season progresses. But, the umpires said, the strike zone created by the system varies from park to park, forcing them to alter their strike zones from park to park. The umpires say they can have yet another strike zone in the parks where they are not monitored.

"It's having an effect on the game," said one umpire, who like all of the umpires who commented in telephone interviews did so only if they would not be identified.

"It's definitely a concern," another umpire said.

Umpires have questioned the QuesTec system for the past year, but the commissioner's office has forced them to try to comply with the QuesTec strike zone. Umpires have been told that if at least 90 percent of their calls do not conform with QuesTec calls, they are guilty of below-standard umpiring.

"I try to call the game I would normally call," one umpire said, "but I think about QuesTec every once in a while. When you start thinking, you're in trouble. The worst feeling an umpire can have is second-guessing yourself. That's what QuesTec does. Umpires say they are losing their confidence."

Another umpire said he and his colleagues had to change their strike zones from QuesTec park to QuesTec park. "For years, you're reacting to what happens; you call what you see,'' he said. "In a QuesTec city, you say, 'What is the machine going to say?' not 'What was that pitch?' Pretty soon you're umpiring a video game, not a baseball game. It affects your mind-set of what you're doing out on the field."

What makes the system worse, this umpire said, is that the strike zone, which is established by the computer operator, varies from park to park, from at-bat to at-bat with the same batter and sometimes even from pitch to pitch.

Sandy Alderson, executive vice president for baseball operations under Commissioner Bud Selig, did not return a telephone call yesterday seeking comment, but on Tuesday he strongly defended the system, telling The Associated Press that "the umpires have never been more accurate and more consistent about the strike zone and the rule book than they are today."

The QuesTec system, which the World Umpires Association has challenged in a grievance that is scheduled to be heard in July, has come under public scrutiny since last Saturday night, when e Arizona pitcher Curt Schilling took a bat to one of the cameras through which the system operates at the Diamondbacks' Bank One Ballpark. His earned run average this season is 4.39 in six starts at home and 1.96 in three starts on the road.

Schilling will most likely be disciplined for his action, but he gained the gratitude of umpires and fellow pitchers who have come to believe that the system has affected umpires' pitch calls.

"We hear it all the time," Al Leiter of the Mets said in Philadelphia before the Mets' game there last night. "There are a number of umpires saying: 'Al, I'm on the computer tonight. It's a computer night.' "

Tom Glavine, also a Mets pitcher, said he had heard similar comments from umpires. Several umpires spoke of being forced to call a narrower strike zone in QuesTec parks.

"I think that's unfair that they're under pressure to call a different game," Glavine said. "To me, either everybody has it or nobody has it. Whether or not it does anything, if there's even the slightest potential that because of it being somewhere, the game's going to be different versus it not being there, that's tough."

Alderson said the system was easy to operate, but the umpires interviewed yesterday disagreed. They said the system's accuracy varied from operator to operator and depended on the way the operators calibrated the system and the way they set the strike zone from a snapshot taken as the first pitch to a batter was on the way to the plate.

Because of the variations, one umpire said, umpires do not know what the dimensions of the strike zone were until 30 minutes after the game.

"It's an exercise in frustration," he said, adding: "You spend your whole career trying to get good enough to be on the major league level and some guy comes along 30 minutes after the game and tells you based on a grainy photograph where the strike zone was."

Because of the differences they say exist, the umpires said, they share information about the different parks where QuesTec is used.

"It's human nature," one umpire said. "If a truck driver is going down the road and sees a cop, he lets everyone know there's a cop. You go to a QuesTec city and you pass on information about it."

Another umpire said, "We also share the despair going from park to park."