In the present experiment we examined the effect of a direct in vitro infusion of progesterone upon spontaneous and amphetamine-stimulated in vitro dopamine (DA) release and post-superfusion DA tissue concentration of corpus striatum tissue fragments from ovariectomized and estrogen-treated female rats. An intermittent infusion of progesterone at a dose of 2 ng/ml produced a significant increase in amphetamine-stimulated DA release and post-superfusion DA tissue concentration compared to similar superfusions infused with medium alone or cholesterol (2 ng/ml). Higher (50 ng/ml) or lower (0.2 ng/ml) doses of progesterone were ineffective and a continuous infusion of progesterone at an identical total concentration to that of the intermittent 2 ng/ml dose inhibited both amphetamine-stimulated DA release and post-superfusion DA tissue concentration. With the exception of 5 alpha DHP (dihydroxyprogesterone) intermittent infusions of various metabolites, a synthetic progestin (R5020) at 2 ng/ml and estradiol at both 0.2 ng/ml and 2 ng/ml failed to modify significantly the amphetamine-stimulated DA response. However, pregnanolone, 5 alpha DHP, R5020 at 2 ng/ml and estradiol at 0.2 ng/ml increased post-superfusion DA tissue concentration to levels comparable to that of progesterone. These results demonstrate that in vitro progesterone can directly alter the amphetamine-stimulated DA release from dopaminergic terminals of corpus striatal tissue fragments. This effect appears quite specific for progesterone as well as for a specific dose and mode of infusion of this gonadal steroid. Moreover, progesterone can exert opposite effects upon the amphetamine-evoked DA release from the corpus striatum as a function of its mode of infusion suggesting a means by which one hormone can differentially alter central nervous system function.