Season 9, Episode 22: The One with the Donor
John Stamos guest stars as Zack, an unsuspecting sperm donor for Chandler and Monica. Zack is unknowlingly invited to dinner as a test to see if he will be a fitting donor. The couple scares him away, though, because of all the random health-related questions. This marks the turning point for Chandler and Monica, and they ultimately decide to adopt a baby instead.