The presence of a transplanted prolactin- and GH-secreting pituitary tumor (Furth MtT·W15) in inbred male rats resulted in increased hypothalamic LHRH and pituitary LH content, decreased serum LH and testosterone concentrations, and very high serum prolactin values. The pituitary tumor also inhibited LH release by the in situ pituitary in response to orchidectomy, or orchidectomy and adrenalectomy, and reduced the LH response to LHRH administration. Testes weight was significantly reduced and adrenal weight was significantly increased in the rats carrying pituitary tumors. These results suggest that the inhibitory effects of the transplanted pituitary tumor on LH and testosterone secretion were affected both a reduction in hypothalamic LHRH release and a reduced responsiveness of the pituitary to LHRH