Our recent findings on the subcellular localization of exogenously administered ¹²⁵I-labeled mouse submaxillary renin in the mouse kidney are summarized herein. ¹²⁵I-labeled renin was given intravenously to anesthetized male ICR mice (6 weeks ) in a dose of 20 ng (2 μCi)/40g body weight and the subcellular distribution was identified by the reaction with a specific antiserum of the mouse submaxillary renin. ¹²⁵I-labeled renin was identified mainly by differential centrifugation in the heavy mitochondrial fraction. With discontinuous sucrose density gradient centrifugation, the radioactivity was most readily detected with 1.2 M and 1.5 M sucrose. Over 90% of the radioactivity was identified in the form of ¹²⁵I-labeled renin with a molecular weight of 38, 000 daltons in each fraction. Light and electron microscopic autoradiography revealed that the ¹²⁵I-labeled renin had accumulated in the apical region of the proximal convoluted tubular cells and was mainly located in the lysosomal granules. All these data taken together indicate that the exogenous renin filtered through the glomerular capillaries is reabsorbed and localized in the lysosomes of the proximal convoluted tubular cells.