The hGH/hCS genes, clustered on chromosome 17 in the 5' to 3' order GH-N, CS-L, CS-A, GH-V and CS-B, show a high degree of sequence identity. The expression product of the GH-V gene is the placental growth hormone, which replaces pituitary GH in maternal blood throughout pregnancy. By means of mRNA competitive hybridization using 32P-labelled and unlabelled 30 bases long oligonucleotides, we first optimized specific hybridization conditions. In situ hybridization was then performed to locate the GH-V mRNA encoding placental growth hormone. The hGH-V gene appears expressed in the placental syncytiotrophoblast. Unlike the CS-A and CS-B genes (both encoding hPL) which are expressed uniformly in the syncytiotrophoblast, the GH-V mRNA is located in a few syncytiotrophoblast cells only.