Mack, who has rushed for 2,989 yards in four years with the Browns, looked straight ahead as he left the courtroom with his wife, Ava, and did not speak to reporters.
"We continue to monitor the case and will review today's court proceedings," NFL spokesman Joe Browne said. "We do not plan to comment further - probably until after Kevin Mack's sentencing."
Offensive tackle Mike Ariey of Green Bay, suspended for four weeks because he tested positive for anabolic steroids, admitted his mistake and said he believed a lot of steroid users in the NFL have not been caught.
"It's tough to talk about it without sounding like I'm giving a bunch of excuses," Ariey was quoted as saying in yesterday's editions of the Bakersfield Californian. "I made a mistake.
"It was supposed to get out of your system. I wanted to take something where I could get strength. I took a chance."
Ariey, 25, said he had taken the advice of a Bakersfield bodybuilder who told him that any traces of steroids would be flushed out of his system before he was tested by the league.
Ariey said he was tested on July 24, about five weeks after he stopped taking steroids.
A book to be published next week alleges that wide receiver Irving Fryar of New England, while playing for the University of Nebraska, used drugs, received cash payments and could deliberately have dropped passes in the 1984 Orange Bowl game.
The book, Big Red Confidential, by Armen Keteyian, a former investigative reporter for Sports Illustrated, describes alleged abuses in the Nebraska football program under coach Tom Osborne.
It includes charges that Fryar used cocaine in college and that he may deliberately have dropped key passes in the Cornhuskers' 31-30 loss to the University of Miami in the '84 Orange Bowl.
It also includes allegations that Fryar and other Nebraska players may have been paid for scoring touchdowns while at the school.
Keteyian declined on Tuesday to comment on the book or his sources.
Fryar's agent, Sherwood Blount, declined to comment on the book. He said Fryar, who is holding out because of a contract dispute, would not comment before seeing the book.