We have investigated the effect of single injections (1 mg/animal i.v.) of prolactin or vehicle, and repeated depot (0.125-1.0 mg/animal/day) or bolus (1.0 mg/animal/day) administration of prolactin or vehicle for 6 days to adult male rats. The density (Bmax) and affinity (Kd) of specific striatal [3H]spiperone binding was not changed by any of the prolactin treatment schedules used. Allogeneic transplants of anterior pituitary glands resulted in an increased concentration of circulating prolactin but did not alter the density or affinity of specific striatal [3H]spiperone binding in male rats, measured 2 weeks following the operation. Prolactin did not displace specific striatal [3H]spiperone binding when incorporated in vitro. Any effect of prolactin on striatal dopamine receptor function appears to be exerted only by high, non-physiological concentrations, and such effects are difficult to reproduce.