Around the onset of labor, uterine sensitivity to oxytocin (OT) increases tremendously. Although this is considered to reflect OT receptor (OTR) augmentation in myometrium, neither spatial expression of OTR nor the level of the receptor message during the course of pregnancy have been investigated at the molecular level. We examined the localization and expression of the OTR in human myometrium by means of in situ hybridization, immunohistochemistry, and Northern and Western blotting. In the term pregnant myometrium, OTR expressing smooth muscle cells are observed diffusely and heterogeneously. Some of the smooth muscle cells were expressed high levels of the receptor at the messenger RNA and protein level, and they were surrounded with cells weakly positive for the OTR or negative. The level of OTR transcripts increased according to the course of pregnancy. The receptor messenger RNA level reached over 300-fold at parturition compared with the nonpregnant myometrium. In the myometrium at 32 weeks of gestation and not in labor, a relatively large amount (about 100-fold) of the receptor message was expressed. In the nonpregnant myometrium, significant amount of the receptor protein was revealed by Western blotting. We also found that the receptor protein was augmented at term and after the onset of labor. These findings indicated that the expression of OTR changes dynamically at the transcription and protein level during pregnancy and that its expression is heterogeneous in the term myometrium.