maf is a family of oncogenes, originally identified from the avian oncogenic retrovirus AS42, which encodes a nuclear bZip transcription factor protein. It has been reported that maf family genes have critical roles in embryological development and cellular differentiation. In this study, the distribution of maf-1 genes, the rat homologues of mafB, was examined in rat kidneys at the embryonic stages from 13 days, gestation (E13) through E21 and then 1, 2, 4, and 8 weeks after birth by in situ hybridization with S-35-labeled antisense riboprobes. The cellular localization was determined using double in situ hybridization. Expression of maf-1 mRNA appeared weakly on E15 and was restricted to glomerular visceral epithelial cells during the pre- and postnatal stages until 2 weeks after birth and then gradually diminished. Double in situ hybridization demonstrated that maf-1 mRNA-positive cells in glomerulus also expressed Pod-1 gene, suggesting that maf-1 mRNA was expressed in the podocyte. These findings suggest that the expression of maf-1 gene may be involved in embryological development and/or differentiation of the kidney.