This paper studies the time-course of the effects of pharmacological administrations of ovarian steroids on the functional state of dopaminergic terminals in the striatum and the limbic forebrain, using the ratio between the contents of dopamine (DA) and its metabolite, L-3,4 dihydroxyphenylacetic acid (DOPAC), as an index of nerve activity. Estradiol produced an increase in the dopaminergic activity of both limbic and striatal neurons, reflected in the high DOPAC/DA ratio observed in both areas. This estrogenic effect was only observed at 4 hours, disappearing in the subsequent times studied. The effect was antagonized by progesterone in both tissues, since a single injection of this steroid to estrogen-pretreated rats restored to control values the estradiol-induced increase, suggesting the existence of negative interactions between both steroids. Furthermore, treatment with progesterone produced also a late decrease of the DOPAC/DA ratio in the striatum, which was observed only in the animals nonpretreated with estrogens.