Lee, who opened the season on the 15-day disabled list with a strained lower abdominal muscle, is on target to return the first week of May.
"Awesome," Lee said of his bullpen session. "It was great."
Mariners manager Don Wakamatsu said Lee had no problems and that the left-hander feels like he can already pitch.
"The last thing we want to do is set him back," Wakamatsu said. "We'll massage it."
The plan going forward is for Lee to throw 45 pitches on Sunday here in Texas. He will take two days off, throw a bullpen Wednesday, then throw in a simulation game next Friday before the Mariners' home game against Detroit.
The hope is to have Lee throw another simulated game three of four days later -- he'll try to throw 55 pitches in both -- then send him out on a rehab assignment and get him up to 75 or 80 pitches before his first start for the Mariners.