The Mas proto-oncogene encodes a G-protein-coupled receptor with the common seven transmembrane domains and may be involved in the actions of angiotensins. Because Mas is highly expressed in testis, we investigated the cell type-specificity and the onset of expression of the gene in this organ. Using an RNase protection assay, it could be shown that neither whole testes nor cultured Sertoli and Leydig cells of 12-day-old mice express Mas mRNA. Mas expression is first detected in 18-day-old mice and thereafter increases continuously until 6 months of age. By in situ hybridization, the expression could be localized to Leydig cells and Sertoli cells, the signals being much more pronounced in the former. A weak signal was detected in primary spermatocytes. The strong ontogenetically controlled and cell type-specific expression of this membrane-bound receptor in testis implicates a role for the Mas proto-oncogene in testis maturation and function.