Adenohypophyseal growth hormone (GH) and prolactin (PRL) cells of suckling mink of both sexes at different stages of lactation were studied by the immunogold method and morphometry for electron microscopy. Hypophyses were perfused with fixative, postfixed in osmium tetroxide, and embedded in Epon 812. Ultrathin sections were labeled by the immunogold method with anti-human GH and anti-human PRL sera. Two PRL cell types were identified based on their morphology and the development of cytoplasmatic organelle. Significant differences in the granular sizes of GH and PRL cells were found between males and females at different stages of suckling. The results suggest that the changes in secretion and storage in GH and PRL cells of suckling mink during lactation depend upon sex and age, and that the granular polymorphism is not an unequivocal criterion for the identification of PRL cells in suckling mink.