Neurons in the adult rat ventromedial hypothalamic nucleus (VMN, 4-6 neurons per brain; 3-7 brains per group) were studied under various hormonal conditions using the single-section Golgi impregnation technique. Intact rats of both sexes and ovariectomized females treated with oil, estrogen or estrogen and progesterone were used. Golgi-impregnated neurons in the VMN were analyzed to determine possible differences in cell body size, number of primary dendrites, number of dendritic branch-points and spine density. The only significant differences found were in spine density. In the VMN of ovariectomized rats given oil, there were significantly fewer spines on primary dendrites than in either estrogen-treated, estrogen plus progesterone-treated or intact female rats. There were no differences between intact male and female rats observed in any parameter. In addition, when spine density of VMN neurons was assessed throughout the estrous cycle, it was determined that spine density was significantly lower at diestrus than proestrus.