The subcellular localization of renin and kallikrein in rat kidney cortex homogenate was investigated using both differential and density gradient centrifugation techniques. Highest specific activity of renin was found in the heavy mitochondrial fraction. Mitochondrial localization of renin was further supported by the behaviour of succinic dehydrogenase. By differential centrifugation, highest specific activity of kallikrein was found in the light mitochondrial fraction, while by density gradient centrifugation kallikrein was almost completely recovered in the lysosomal fraction. Lysosomal localization of kallikrein is further supported by the behaviour of acid phosphatase. The different subcellular localizations of renin and kallikrein are confirmed and the suggestion that kallikrein is located in the lysosomes is advanced.