Estrogen treatment of ovariectomized female rats for a week decreased plasma osmolality from 294 to 278 mosmol/kgH2O, but recordings from the whole animal preparation under urethan anesthesia failed to detect any change in the activity of 76 vasopressinergic neurons in the supraoptic nucleus (SON) of the hypothalamus. In brain slice preparations, neuronal discharge of 74 presumed vasopressinergic SON cells increased linearly as the osmolality of the perfusate was increased from 278 to 320 mosmol/kgH2O, each 10 mosmol/kgH2O fraction corresponding to a 0.7-Hz change. This osmolality-neuronal activity relationship was similar in the slices taken from the estrogen-treated and the control ovariectomized animals, except that the neurons in the former fired twice as frequently as those in the control throughout the osmolality range. This difference explains the estrogen-induced decrease in the osmolality threshold to elicit antidiuresis and the resultant water retention in estrogen-treated animals. It was reasoned that the estrogen-induced neuronal activation was offset by the decreased plasma osmolality in the animal as a whole.