The participation of gonadal steroid hormones in regulation of the vasoactive intestinal peptide (VIP) gene expression in the hypothalamus was studied using a quantitative densitometric hybridization assay. In the female rat the levels of VIP mRNA were found to be significantly decreased following ovariectomy (4.41 ± 0.7 arbitrary units of absorbance vs. 8.52 ± 0.18). This decrease was largely reversed after three days of treatment with estradiol dibenzoate. In contrast to the female rats, no significant change in VIP mRNA levels was observed in the male rats, following orchidectomy. These results suggest a sexual dimorphism with regard to the steroid regulation of hypothalamic VIP gene expression in the rat.