The renin regulatory locus (Rnr) is a genetic element governing mouse submaxillary gland (SMG) renin levels. A 45, DDO dalton polypeptide detectable after in vitro translation of mouse SMG mRNA has been identified by genetic and physical criteria as SMG renin. A cDNA recombinant clone specific for SMG renin has been isolated and used to demonstrate that the previously described genetic regulation of SMG renin levels is manifest at the level of renin mRNA concentration. The renin cDNA clone has also been used in Southern blot analyses to study the organization of homologous DNA sequences in strains carrying different alleles at the Rnr locus. Restriction digest patterns of high renin strains (RnP) are characteristically distinct from patterns observed for low renin strains (Rnrb) and are suggestive of a structural gene duplication at the chromosome 1 locus in high renin strains. HoW-ever, gene dosage cannot account for the increased levels in high renin strains, since SMG renin levels in RnFand those in Rnrb may differ up to lOD-fold.