More pronounced hyperprolactinemia occurred in the male nude mice bearing a transplantable rat pituitary tumor than in the female nude mice, although the hyperprolactinemia in rats bearing the identical tumors was more prominent in the females than in the males. PRL levels in castrated nude mice of both sexes bearing tumor were almost the same as those in the intact female nude mice. PRL levels in the tumor tissues in each group paralleled relative abundance of PRL-secreting cells in the tumors. Supplement with testosterone or estrogen to the castrated mice recovered the PRL concentrations to the same or higher levels than those in intact male mice. Reproductive organs in the tumor-bearing male nude mice showed no alterations, whereas those in the female nude mice were markedly atrophic. Tumors had receptors for estrogen but not for androgen. Insufficient estrogen secretion due to the ovarian atrophy caused by the PRL from the tumors seemed responsible for the failure in growth of PRL-secreting tumor cells and consequent low PRL production by the tumor.