Changes in OR mRNA expression in the ventromedial hypothalamus (VMH) in relation to the estrous cycle were measured by in situ hybridization with a rat oxytocin receptor (OR) probe. Binding studies have localized ORs to various brain regions, and have detected a high density of receptors in the VMH, a nucleus containing large numbers of estrogen responsive neurons. Previous studies in this lab have reported a significant increase in OR mRNA expression in the VMH in ovariectomized rats treated with estrogen. The present study was designed to determine whether changes in steroid hormone levels across the estrous cycle result in induction of OR mRNA expression. Autoradiographic studies revealed differences in OR mRNA expression in the rostral and caudal as well as medial and lateral aspects of the VMH. OR mRNA levels were highest in the caudal portion of the vIVMH on the afternoon (16:00 hr) of proestrus. The rostral region exhibited a high level of expression in the ventrolateral region of the VMH on the morning (9:00 hr) of proestrus and in the dorsomedial region of the VMH on the afternoon of proestrus. Little or no OR mRNA expression was evident in the rostral or caudal VMH on the morning or evening of diestrus. These results support previous findings which showed a regulation of OR binding by gonadal steroids and suggest that this may be due to altered expression of the OR gene. These effects suggest a possible role of ORs in the oxytocin stimulated release of luteinizing hormone.