Direct prolactin (PRL) effects on dopamine (DA) release from fragments of rat striatal tissues were studied by the in vitro superfusion technique. The DA collected in the superfusates was quantified by a radio-enzymatic assay. In the present experiments, intact and one-week gonadectomized adult male and female rats were used. The spontaneous release of DA from male and female rat striatal tissues was relatively stable after a 60 min equilibration period. The basal release rate of DA from striatal tissues from intact males infused only with medium over a 76 min period was 22.5 +/- 0.7 pg/mg/min (X +/- SE of 4 experiments). Infusion of highly purified PRL (NIAMDD-rPRL-I-5) at a concentration of 1 microgram/ml for 24 min significantly augmented DA release from striatal tissues from intact and castrated males (167 +/- 15% (n=6) and 203 +/- 32% (n=4), respectively) over pre-infusion basal levels. Infusion with heat-denatured PRL or other proteins had no effect on DA release from intact male striatal tissues. In parallel experiments, infusion of PRL at 0.1, 1 or 10 micrograms/ml concentrations for 24 min had no effect on the basal release of DA from striatal tissues from intact and ovariectomized females. These results demonstrate that an absolute sexual difference exists in the responsiveness of rat striatal tissues to PRL infused in vitro. Interestingly, this sexual difference was not modified by short-term castration.