A genomic clone was used to generate a rat oxytocin receptor (OTR) probe for in situ hybridization studies in order to monitor changes in expression of OTR mRNA in the CNS of male and female rats in brain regions reported to be rich in OTR binding. This probe predominantly detects expression in the ventromedial hypothalamus (VMH). Quantitatively greater expression of OTR mRNA was more evident in the male VMH than in that of the female. OTR mRNA expression in the VMH is also enhanced in gonadectomized rats treated with either estrogen or testosterone. Previous studies of VMH OTR mRNA expression during the estrous cycle revealed a role of estrogen in proestrous OTR mRNA expression and also provided evidence for progesterone's modulatory effect on OTR expression. A related study which examined OTR mRNA expression during gestation, parturition, and lactation further supported the regulation of the OTR by gonadal steroids, as a pronounced induction of expression occurred at parturition when estrogen levels were highest. These results support previous studies which have documented the regulation of OTR binding by gonadal steroids and suggest that this regulation may be the result of altered expression of the OTR gene. Induction of OTRs in anatomically distinct regions of the VMH may be important in promoting OT'ergic neurotransmission required for sexual behavior or the induction of gonadotropin release by OT, which requires estrogen and progesterone priming. The anatomical variation in OTR mRNA localization and its restricted detection in the VMH by this probe suggests potential CNS OTR heterogeneity.