We examined the effects of ovarian hormones on dendritic architecture of neurons in the ventromedial nucleus of the hypothalamus in female Syrian hamsters. Treatment with 10 micrograms of estradiol benzoate for two days, or estradiol benzoate for two days followed by an injection of 500 micrograms of progesterone, increased the total dendritic length of ventromedial nucleus neurons by almost 50% compared with neurons from the ventromedial nucleus of ovariectomized, oil-treated females. Neurons in a control region, the dorsomedial nucleus of the hypothalamus, were unaffected by these hormone treatments. These results demonstrate that steroids can induce changes in dendritic structure within 48 hr, suggesting that such morphological reconfiguration of hypothalamic neurons may underlie variations in behavior associated with the female's 4-day estrous cycle.