The central part of the medial preoptic nucleus (MPNc), the encapsulated part of the bed nucleus of the stria terminalis (BSTe), and the posterodorsal part of the medial nucleus of the amygdala (MeAp) are all thought to be involved in the neural control of female reproductive behavior, as well as other neuroendocrine mechanisms. Although the developmental importance of gonadal steroids during the perinatal period on these sexual dimorphisms is well known, an understanding of possible activational effects on these cell groups of circulating gonadal steroids in the adult is less clear. In the present study we evaluated the number of cholecystokinin (CCK)-immunoreactive cells present within MPNc, BSTe, and MeAp of regularly cycling female rats over the estrous cycle. In addition, the effects of ovariectomy and estrogen replacement on CCK staining were also examined. The number of CCK-immunoreactive cells within each cell group varied over the estrous cycle with the fewest cells present in animals sacrificed while in diestrus. Proestrous female rats showed a greater number of cells within each nucleus, while intermediate numbers were found for animals in estrus. These changes appear to be due, at least in part, to changes in levels of circulating estrogen, since subcutaneous implants of estradiol prevented the decline in the number of CCK-stained cells within MPNc, BSTe, and MeAp that was seen in untreated, ovariectomized female rats. Thus, the present findings support the hypothesis that levels of CCK within cells of these three sexually dimorphic cell groups are regulated by circulating gonadal steroids within a physiologically relevant time frame and may possibly contribute to the activation of female reproductive behavior as well as other neuroendocrine functions.