The grievance to overturn Alex Rodriguez's 211-game suspension began Monday before arbitrator Fredric Horowitz.
The New York Yankees third baseman was wearing a business suit and accompanied by lawyers when he arrived for the session at Major League Baseball's headquarters in midtown Manhattan.
A three-time AL MVP, Rodriguez was suspended Aug. 5 for alleged violations of baseball's drug agreement and labor contract. Because he's a first offender under the drug program and the players' association filed a grievance to force an appeal, a suspension can't start until it is upheld by an arbitrator.
The union argues the discipline is without just cause and is excessive. If the case doesn't settle, a decision by Horowitz is expected this winter.
Rodriguez was among 14 players penalized by MLB this year following the sport's investigation of Biogenesis of America, a closed anti-aging clinic in Coral Gables, Fla., accused of distributing banned performance-enhancing drugs. The others accepted their penalties, including former NL MVP Ryan Braun, who missed the season's final 65 games.
Biogenesis head Anthony Bosch was at the hearing to testify - a publicist released a photo of him in a hallway outside MLB's conference rooms.
In Florida, state authorities said they had started a criminal investigation of the clinic.
''A subpoena was issued for documents, and we are looking into several areas of state interest,'' said Ed Griffith, spokesman for Miami-Dade County State Attorney Katherine Fernandez Rundle.
Existence of the state prosecutors' criminal subpoena was disclosed at a recent hearing in MLB's lawsuit in Miami against Biogenesis, also pending in Miami-Dade County court. It's not known whom the subpoena targeted or what specific documents are being sought.
A federal probe involves the sources of drugs the clinic is accused of selling to players.