The effects of ovariectomy on the ultrastructure of neurons within the SCH nuclei of the rabbit were investigated. All SCH neurons of the intact controls were similar in that they showed no morphological evidence of enhanced neurosecretory activity. In contrast, two distinct types of neurons were present within the SCH nuclei of the ovariectomized animals. The small, Type I neurons were ultrastructurally identical to the SCH neurons of the intact controls. The larger, Type II neurons, on the other hand, clearly presented structural evidence for elevated intraneuronal synthesis. To ensure that the changes which characterized the Type II neurons were due, at least in part, to the removal of ovarian steroids following ovariectomy and not to surgical stress alone, the SCH nuclei of laparotomized rabbits were also studied. Although the same two neuron types were observed, the number of Type II neurons in the laparotomized controls was significantly smaller (p < 0.001) than the number in the ovariectomized group. These findings suggest that Type II neurons probably represent a functional state of the smaller, non-active neurons of the intact controls. In addition, the ultrastructural features that distinguish Type II neurons are positively related to the endocrine state of the rabbit and, therefore, may represent the hypothalamic synthesis of the releasing hormones that control its gonadotropic activity.