Sex differences in basal extracellular striatal dopamine concentrations in gonadectomized male and female rats have been reported previously. In the current experiment, estrous cycle-dependent variation, sex differences and the effect of gonadectomy on extracellular striatal dopamine concentrations were determined using quantitative microdialysis. Female rats were found to have significantly higher extracellular striatal dopamine concentrations in proestrus and estrus than in diestrus or after ovariectomy. In contrast, castration of male rats had no effect on extracellular striatal dopamine concentrations. Thus, endogenous ovarian hormones, but not testicular hormones, modulate extracellular striatal dopamine concentrations in rats.