OP-1, a bone morphogenetic protein (BMP) in the TGF-beta superfamily, is expressed at high levels in the kidney and in the endometrium of the uterus of non-pregnant mice. During pregnancy the OP-1 mRNA in the endometrium rapidly declined at 4 dpc. Thereafter, OP-1 transcripts were detected in the trophoblastic giant cells of the placenta and the fetal tissues. The uterine OP-1 mRNA downregulation could be mimicked by administration of 17 beta-estradiol but not by progesterone to non-pregnant animals. In contrast, OP-1 mRNA expression in kidneys and ovaries was not affected by pregnancy or estrogen treatment. The selective effect of estrogen on OP-1 mRNA in the uterus suggests that OP-1 expression is regulated by tissue specific mechanisms.