The electrical activity of caudate neurons and their response to iontophoretically applied dopamine (DA) were tested at different time intervals before and after i.m. injection of estradiol benzoate in rats ovariectomized one week previously. One hundred and three neurons were recorded before estradiol injection and, of these, 25% were spontaneously active and 75% were driven by glutamate. The firing activity of the majority (86%) of the neurons was inhibited on application of DA. At 2--6 after the estradiol injections, the proportion of spontaneously active neurons was elevated to 87% and their firing rate was dramatically increased. Concomitantly, the proportion of cells affected by DA decreased to 39%; moreover, the majority of cells responsive to DA was now excited following it. We conclude that estrogens are able to influence the global activity of caudate neurons and to reverse their sensitivity to DA. This action may be partly mediated by the pituitary since it was not observed in ovariectomized rats which were also hypophysectomized.