The present study was concerned with the effects of a transplantable pituitary tumor secreting prolactin (PRL) and adrenocorticotrophin (ACTH) on the levels of LH and FSH in peripheral plasma and on the hypothalamic release of LH-RH and dopamine in the male rat. Male rats of the same age not inoculated with the tumor served as controls. Hypophysial stalk blood was collected from urethane-anesthetized rats 4-5 weeks after tumor inoculation to measure their LH-RH and dopamine content. A peripheral blood sample was withdrawn from the animals just before sectioning the hypophysial stalk to measure their content of LH, FSH and PRL. It was found that in the tumor-bearing rats the levels of PRL increased 17-fold, whereas plasma levels of LH and FSH decreased by 45 and 70% respectively, when compared with the control rats. In the tumor-bearing rats, the secretion rate of dopamine in hypophysial stalk plasma increased from 1.4 to 4.1 ng/h, whereas the secretion rate of LH-RH decreased from 122 to 61 pg/h. However, when at the time of tumor inoculation adrenalectomy was performed, the tumor did not decrease plasma levels of LH and FSH and the secretion of LH-RH into hypophysial stalk blood any longer. The effect of the tumor on hypothalamic dopamine secretion was, however, still present in the adrenalectomized rats. It is concluded that the effect of the PRL- and ACTH-secreting pituitary tumor on plasma levels of LH and FSH requires the presence of the adrenal gland and that this effect is mediated through an inhibition of the hypothalamic release of LH-RH. Furthermore, this tumor increases the hypothalamic release of dopamine independent of the presence of the adrenal gland.