[quoted from wikipedia Gates McFadden article]
McFadden was fired and replaced by actress Diana Muldaur as the Enterprise's Chief Medical Officer for the second season. Muldaur's character, Dr. Katherine Pulaski, didn't make it to the third season. Roddenberry described Muldaur as "a most talented actress", and said that the decision "to let her go was made solely because the hoped-for chemistry between her and the rest of the starship cast did not develop." McFadden was approached to return for the third season. At first she was hesitant, but after a phone call from co-star Patrick Stewart, McFadden was persuaded to reprise her role.