To delineate tissue sites of renin synthesis the levels of renin enzyme activity and renin mRNA sequence were measured in various organs of adult male outbred Swiss mice. Blots containing poly (A)+ RNA from each organ were hybridized to the renin cDNA probe ID-2 labelled by nick translation to quantitate renin mRNA sequence. Densitometric analysis of these blots and similar blots containing total organ RNA in place of poly (A)+ RNA showed that the relative organ contents of RNA sequence complementary to renin cDNA could be ordered as follows: submandibular gland greater than testis greater than kidney greater than adrenal greater than heart; spleen and liver were unreactive. Measurements of renin enzyme activity in these organs were in general agreement with the hybridization results. These data document the widespread synthesis of renin in organs of the mouse and suggest a general biological role for renin. Identification of the specific cells expressing renin mRNA will further our understanding of tissue-specific expression of the renin genes.