Two estrogen receptor (ER) isoforms, ER and ER, have been described. However, no information is available in any species regarding the comparison of ER and ER levels in pregnant intrauterine tissues. We investigated 1) distribution of ER and ER mRNA in myometrium, amnion, choriodecidua, and placenta; 2) their abundance in intrauterine tissues at term not in labor (NIL) and in spontaneous term labor (STL); and 3) immunolocalization of ER and ER in pregnant rhesus monkey myometrium. Myometrium, amnion, choriodecidua, and placenta were obtained at cesarean section from monkeys in STL at 156-166 days gestational age (GA) (n = 4) and from control monkeys NIL at 140-152 days GA (n = 4). RT-PCR was conducted to determine ER and ER and glyceraldehyde-3-phosphate dehydrogenase mRNA abundance in four intrauterine tissues of the pregnant rhesus monkey. The cloned ER PCR fragment was subjected to sequence analysis. ER and ER were localized in the myometrium by immunohistochemistry. We demonstrated that 1) rhesus monkey ER shares >97% identity with human ER in the region sequenced; 2) both ERs were expressed in myometrium, amnion, and choriodecidua but not in placenta in the current study; 3) ER and ER were differentially distributed in myometrium and amnion; 4) ER and ER were immunolocalized in myometrial smooth cells and smooth muscle and endothelial cells of the myometrial blood vessels. The biological significance of these quantitative differences in ER subtypes merits further study.