Studies were undertaken to (1) examine the effects of hyperprolactinemia on the frequency and amplitude of pulses of LH, and (2) determine if changes in pituitary sensitivity to LHRH were involved in the prolactin-induced suppression of LH secretion. Rats were bilaterally ovariectomized (day 0). Ovine prolactin (4 mg/kg body weight, subcutaneously) or vehicle was administered every 8 h beginning at 09.00 h on day 4 after ovariectomy and continuing until 09.00 h on day 6. On day 6, between 07.00 and 09.00 h all animals received a right atrial cannula, using ether anesthesia. In experiment I blood samples were taken at 10-min intervals beginning at 12.00 h on day 6, for a total of 180 min. To test the effect of hyperprolactinemia on pituitary responsiveness (experiment II) animals received an intravenous injection of LHRH (25 ng/100 g body weight) after the 180-min and again after the 240-min sample. Blood was drawn every 10 min for a total of 300 min. Serum was assayed for LH. Hyperprolactinemia altered the pattern of pulsatile secretion of LH. Treatment with ovine prolactin produced a decrease in both the frequency and amplitude of the LH pulses compared to values found in control animals. However, no differences in pituitary responsiveness between hyperprolactinemic and control animals were found at the dose of LHRH given. Thus, the prolactin-induced suppression of pulsatile secretion of LH was not apparently a result of alterations in the sensitivity of the pituitary to LHRH. From these studies we suggest that hyperprolactinemia directly affects a hypothalamic site which ultimately alters the LHRH pulse generator, thereby changing the secretion of LHRH.