Gonadotropic activity is present in the placenta of various mammals including humans. Human and equine chorionic gonadotropin (CG) are well characterized and known to be glycoproteinhormones, as are pituitary LH, TSH and FSH. In the rat, however, the placental gonadotropin molecule has not been identified, and in fact the existence of CG in the rodent is still controversial. In the present study, the placental expression of mRNA for CG-like molecules was investigated by PCR with mixed primers designed for the CG-β subunit based on the amino acid sequences common to the rat LH-β, human LH-β, equine LH-β and human CG-β subunits and for the α subunit designed from the rat LH-α subunit. PCR with mRNA extracted from the placenta (day 10 of pregnancy) amplified several bands including cDNA identical to part of rat LH-α and -β. Furthermore, expression of these genes was confirmed in the placenta on days 10, 12, 14, 16, 18 and 20 of pregnancy by PCR with another set of primers specific to rat LH-α and -β and by Southern hybridization with 32P-labeled cDNAs for LH-α and -β. The specific bands for these subunit mRNAs were amplified in the placenta, as well as in the positive control i.e. the anterior pituitary gland, but were not detected in other tissues including decidual tissue, liver, spleen, intestine, brain and kidney. Expression of the mRNAs was observed in both the junctional and labyrinth zones of the placental tissue. The rat choriocarcinoma cell line Rcho-1 also expressed LH-α but not LH-β regardless of the stage of differentiation in vitro. Here we show the expression of LH-α and -β mRNAs in the rat placenta.