In the present experiments, we examined the effect of castration upon two indices of nigro-striatal dopaminergic function in the male rat. In Experiment I, differences in spontaneous locomotor behavioral activity between intact and castrated male rats were examined. The total distance traveled, horizontal activity and mean revolutions of castrated male rats were significantly greater than that of intact males. No significant differences between intact and castrated males were obtained for vertical activity. In Experiment 11, the spontaneous in vifrodopamine release from the corpus striatum of intact and castrated rats as sampled during the light-phase (1500 h) and dark-phase (2400 h) of the photoperiod was examined. At both time periods, the spontaneous in vifrodopamine release of castrated males was significantly greater than that of intact males. Both intact and castrated males showed statistically significant increases in dopamine release at the 2400 h compared to the 1500 h time period. To examine if testicular hormones were responsible for these castration induced changes in dopamine release, in Experiment Ill we treated castrated male rats with testosterone propionate. Administration of testosterone propionate (0.1 mg/day x 5 days) significantly reduced in vifro dopamine release compared to untreated or castrated male rats receiving vehicle treatment. These results demonstrate that testicular hormones, most likely testosterone, have a markedly suppressive effect upon the nigro-striatal dopaminergic system as evidenced from changes in spontaneous behavioral activity and in vifro dopamine release.Within our laboratory, we have been interested in examining the modulatory effects of gonadal steroid hormones upon the nigrostriatal dopaminergic system of the female rat (1, 2). Our results, as well as those from other laboratories, have demonstrated that the corpus striatum (CS) is an important target site of the CNS for both estrogen and progesterone (3-6). The role of the male gonadal steroid hormone, testosterone, upon the nigro-striatal dopaminergic system has received relatively less attention. Indirect indices of dopaminergic activity as determined with amphetamine-stimulated behavioral measures have indicated that testosterone attenuates the effects of amphetamine on locomotor activity and stereotyped behavior (7-9) and the duration of specific amphetamine-stimulated behaviors in castrated male rats are prolonged compared to those of intact and castrated plus testosterone-treated male rats (10). In addition, castration reduces and testosterone reinstates yawning behavior in male rats (1 l), a behavior, which in part, appears to be under control of the dopaminergic system (12). These results suggest that, similar to that of the female, the nigro-striatal dopaminergic system of the male rat is also subject to the influences of gonadal steroids. More specifically, it would appear that testicular steroids exert a predominantly suppressive effect upon dopaminergic activity in the male rat.In this report...