The expression of oxytocin receptor mRNA in the human uterus was investigated by polymerase chain reaction after mRNA reverse transcription; in addition, the mRNA was localized by means of in situ hybridization histochemistry. Strong positive signals were observed in myometrial cells at parturition. In nonpregnant human uterine endometrium, oxytocin receptor mRNA was expressed in glandular epithelial cells, but was not observed in stromal cells. The signal in the epithelial cells was stronger at the time of ovulation than during any other phase. The sequential changes in oxytocin receptor mRNA expression in nonpregnant endometrium suggest some alternative biological function of oxytocin in endometrium during the menstrual cycle.