The gene, Wh, causing anophthalmia in the Syrian hamster, Mesocricetus auratus, is a highly pleiotropic gene which has profound effects upon eye development, pigmentation, and reproduction. Hamsters homozygous for this gene also possess abnormalities in testicular size and composition which may be related solely to the lack of the visual pathway acting by means of altered pituitary function. The objective of this study was to determine whether morphological abnormalities existed in hypophyses of male hamsters homozygous for the mutant gene Wh. Accordingly, hypophyses from 10 normal, 10 heterozygous, 10 homozygous mutant, and 5 normal enucleated animals from the AN/As-Wh strain were compared at the light and electron microscopic level. At the light microscopic level, glands from anophthalmic individuals contained 33% fewer cells and many of the cells present either resembled signet ring cells or folliculo stellate cells. At the electron microscopic level, many cells from Wh homozygotes were greatly enlarged, contained only a few organelles and few to no secretory granules. In addition to the glandular cell types, agranular follicular cells were prevalent. Cells with numerous cilia, basal bodies, and a 9 + 2 microtubule configuration were also found within mutant glands. Since pituitaries from hamsters homozygous for Wh displayed far different morphological characteristics than did pituitaries from normal, heterozygous, or normal enucleated animals, it is postulated that either the gene (Wh) acts to alter cellular differentiation of the embryonic hyophysis or that the gene causes abnormal dedifferentiation in the adult.