Twenty pituitary glands, harbouring prolactin cell adenomas, obtained from autopsy of male and female patients of various ages and dying of different diseases have been investigated by the immunoperoxidase technique in order to reveal the morphologic changes of prolactin cells in the nontumorous protions of the anterior lobes. For comparison, 40 nontumorous pituitary glands and 20 pituitaries lodging adenomas not consisting of prolactin cells have also been studied. In all four prolactin cell adenoma cases, the anterior lobes contained numberous prolactin cells outside the tumors, indicating a lack of involution of prolactin cells in the nontumorous portions of the pituitary glands.