Fetuses were examined to produce a developmental profile of renin expression in the kidneys and adrenal glands in single renin gene and two renin gene strains of mice. Sites of renin expression were detected by in situ hybridization using an ³⁵S-labeled antisense RNA probe complimentary to the renin cDNA. Accumulation of renin transcripts in the adrenal gland reached a maximum at 15.5 days post coitum for all strains examined, but declined to undetectable levels by birth in one gene strains, while in two gene strains, the levels of renin transcripts lessened and by birth became limited to the developing inner cortex. Kidney renin transcripts were first detected at 14.5 days post coitum in the newly developing arteries in fetuses of both genotypes of mice. As the renal arterial tree developed, renin mRNA containing cells were progressively localized to more distal blood vessels and finally to the specialized cells of the afferent arteriole (juxtaglomerular cells). These results were confirmed by examining the localization of immunoreactive T antigen in transgenic fetuses. These mice carried a transgene which placed the SV40 T antigen structural gene under control of renin regulatory elements. Expression of T antigen occurred at the same sites in the kidneys and adrenal glands as renin mRNA. Fur-, thermore, in strains with two renin genes, primer extension analysis indicated transcripts from both genes were present in equal proportion in combined kidney and adrenal gland extracts of total RNA. These transcripts were full length in size. The transient localization of renin mRNA in cells of the fetal intrarenal arteries is consistent with the notion that renin may be a useful marker for the developing renal vasculature.