Hyperprolactinaemia induced by pituitary isografts in male host mice was confirmed by radioimmunoassay, but plasma testosterone levels determined by radioimmunoassay in these mice showed no changes. Immunoenzyme electron microscopic observations revealed large spherical-shaped immunoreactive prolactin granules in pituitary grafts in male hosts, regardless of the sex of the donor mice, indicating the disappearance of sexual dimorphism in prolactin-producing cells in hyperprolactinaemic mice. In hyperprolactinaemic host mice the male accessory sex glands, particularly the seminal vesicle and the ventral prostate, exhibited considerable proliferation and significant increase in weight. These phenomena do not seem to be mediated by the increased action of testosterone. Such biological effects in host mice were much greater when the donor was female rather than male, and were more noticeable in C57BL mice than in C3H mice.