Serum LH changes in response to LH-releasing hormone (LHRH) injection were measured in young (4-6 month-old) proestrous, estrous, and second-day-diestrous rats and in aged(23-30-month-old) constant estrous and irregular pseudopregnant (prolonged diestrus) female Long-Evans rats. Serum LH was measured by radioimmunoassay in serial blood samples taken before and at 15, 30 and 60 min after injection of 0, 5, 50 or 500 ng of LHRH. Serum LH also was assayed in similar groups of rats acutely pretreated with either 20 mug of estradiol benzoate (EB) or 5 mg of progesterone. Aged rats showed a smaller increase in serum LH following LHRH injection than the young groups. Within the young groups, the magnitude of response was greater in proestrous and estrous rats than in the diestrous animals. Pretreatment with EB resulted in increased serum LH after LHRH injection in young estrous and diestrous rats and in aged constant estrous rats, but did not alter serum LH levels in the aged pseudopregnant rats. Although the inccrease in serum LH after LHRH injection was greater in all young groups given progesterone pretreatment, progesterone pretreatment did not affect LHRH responsiveness in either of the aged groups. These results suggest that with aging, the pituitary becomes less capable of relasing LH in response to acute LHRH stimulation, and that the pituitaries of aged rats are less responsive to gonadal steroid sensitization to LHRH stimulation.