Effects of gonadal steroids on fos expression in the adult rat ventromedial hypothalamus (VMH) and uterus were examined using molecular hybridization and immunocytochemical techniques. Adult, ovariectomized rats were treated with estrogen (E), progesterone (P), or estrogen followed 24 h later with progesterone (E + P) and sacrificed 1-24 h after treatment. Slot blots revealed an approximately twofold increase in uterine c-fos mRNA within 3 h after receiving 10 mug E. Likewise, an increase in fos-like immunoreactivity (IR) within lumenal and glandular epithelial cells was detected at 3 h, appeared maximal at 6 h and was much reduced by 24 h after E treatment. The induction was both steroid and dose dependent-no induction of fos-like IR was observed following administration of 1 mg P, in response to a low dose of E (0.25 mug/100 g body wt), or when the low dose of E was followed 24 h later with 1 mg P. In addition, the effect was cell-specific-in no case was induction of fos-like IR within stromal or myometrial cells observed. In contrast with the uterus, no effect of E on levels of c-fos mRNA (3 or 6 h after treatment) in the VMH was observed. Sections through areas of the mediobasal hypothalamus were examined for the expression and regulation of fos-like IR. No effects of E, P, or E + P on the number of fos-like IR cells within the ventromedial nucleus of the hypothalamus, tha arcuate nucleus of the hypothalamus, or the medial amygdala were observed. Furthermore, no effects of E, P, or E + P on the expression of fos-like IR within other areas of the forebrain included in these sections were readily apparent. Many fos-like IR cells were consistently detected, however, within several areas of the brain including the arcuate nucleus, the cingulate cortex, the pyriform cortex, the endopyriform nucleus, the suprachiasmatic nucleus, the medial amygdala, and the dorsomedial nucleus of the hypothalamus. Immunoreactive cells in the paraventricular nucleus of the thalamus and in the lateral habenula were also frequently observed. For comparison, intense fos-like IR in the hippocampal formation and in the paraventricular nucleus of the hypothalamus was observed following metrazol-induced seizure activity and water deprivation, respectively. These data are consistent with the hypothesis that estrogenic effects on uterine epithelial cells may be mediated via the induction of fos. In contrast, evidence that estrogenic effects on adult VMH neurons are mediated by fos was not observed.