Ovariectomized female rats injected with 17 beta-estradiol (100 ng, s.c.) showed, as previously observed, an increase of the dopamine (DA) metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) with no change of DA concentrations in the striatum. This increase was observed as soon as 15 min following the injection while plasma estradiol reached a peak of 78 pg/ml after 5 min and was significantly elevated until 45 min to ultimately return to control values at 60 min. We observed no significant change of the inhibition constants of high- and low-affinity D2 DA agonist binding sites and of the sum of high + low agonist DAergic agonist binding densities as detected by apomorphine competition of [3H]spiperone binding. By contrast, a significant conversion of high into low agonist affinity binding states was seen at 15 min (38.6% of conversion, P less than 0.05) and 30 min (40.0% of conversion, P less than 0.01) after the acute physiological steroid injection. Thus, very small doses of estradiol were able to rapidly increase DA turnover and modulate the striatal agonist affinity states of the D2 DA receptor. This effect of estradiol is probably non-genomic, presynaptic and may involve a membrane effect at the DA autoreceptor level.