The hypothesis that 17 beta-estradiol suppresses dopamine secretion into hypophysial portal blood was tested. Portal plasma concentrations of dopamine were significantly lower in proestrous rats (1.0 +/- 0.1 ng/ml; mean +/- SE) than in estrous rats (1.9 +/- 0.38 ng/ml). To deplete the animal of endogenous steroid hormones, proestrous rats were adrenalectomized (Adx) and ovariectomized (Ovx). Twenty-four hours later, hypophysial portal blood was collected for 60 min, and the plasma from this blood was analyzed for dopamine. Arterial plasma from these rats was assayed for 17 beta-estradiol and progesterone. The concentrations of dopamine in the portal plasma of sham-operated rats and bilaterally Adx-Ovx rats were similar to those in estrous animals. The concentration of dopamine in portal plasma of Adx-Ovs rats injected 24 h earlier with 50 micrograms 17 beta-estradiol was 1.0 +/- 0.31 ng/ml, which was comparable to that in proestrous animals but less than that in the estrous rats. The concentrations of 17 beta-estradiol in arterial plasma were as follows: 24 +/- 8.3 pg/ml in proestrous rats, 40 +/- 2.9 pg/ml in estrous rats, 10 +/- 1.3 pg/ml in Adx-ovx rats, and 96 +/- 17.3 pg/ml in Adx-Ovx rats injected with 50 micrograms 17 beta-estradiol. Twenty-four hours after injection of 25 micrograms 17beta-extradiol into Adx-Ovx rats, the plasma 17beta-estradiol levels were 51 +/- 7.4 pg/ml, and the dopamine concentrations in portal plasma were 1.9 +/- 0.57 ng/ml. It is concluded that an acute effect of 17 beta-estradiol is suppression of hypothalamic secretion of dopamine into hypophysial portal blood.