Summary The body weight, kidney weight, renin content, and renin concentration of kidneys were determined in juvenile and adult male and female rats of the Wistar and the Sprague-Dawley strains. In juvenile rats, there was no sex difference in the various parameters; a strain difference was noted, however, as body and kidney weights were smaller and renin concentration higher in Wistar than in Sprague-Dawley rats. In adult animals a sex difference was observed, male rats of both strains having a higher body and kidney weight than females. Renin concentration in males was generally lower than in females. A negative correlation between kidney weight and renin concentration was found in adult rats, independent of sex and strain. It is concluded that renin stores in the glomeruli of males and females are similar in both sexes and strains.