The effect of prolactin (PRL) on luteinizing hormone (LH) pulsation characteristics in intact and castrate male rabbits was investigated. Eight groups of animals were studied. Two groups of castrate and two groups of adult male rabbits served as controls to establish baseline pulsation characteristics. Preliminary studies showed that a sampling interval of 3–4 min was optimal. One group of castrate and one group of intact rabbits were sampled at this frequency for 60 min; two additional groups were sampled for 240 min. With these data determined, four groups were used in paired studies: two castrate and two intact groups received either a bolus injection intravenously of saline or PRL (20 µg), followed by a constant infusion of saline or PRL (50 µg/min) for 3.5 h. Blood was sampled every 3 min for the last 1.5 h of infusion. Serum LH was measured on each sample; rabbit PRL was measured on pooled samples. In selected samples bovine PRL was also measured. The LH pulsation characteristics of the 3 groups of castrate and intact controls (unper-fused and saline-perfused) were not statistically different and the data were pooled. The PRL-treated animals were compared to the controls. Castration increased LH mean concentration, pulse amplitude and duration. PRL infusion decreased mean LH concentration and pulse amplitude in both intact and castrate animals. Pulse duration was decreased significantly in castrate, but not in intact, animals by PRL infusion. We conclude that, in the adult male rabbit, PRL modulates LH pulsation. This action of PRL must occur either directly at the pituitary or more probably through an influence on gonadotropin-releasing hormone pulsation at the hypothala