The activities of the lysosomal endopeptidase cathepsin B (cath B; CZB-Ala-Arg-Arg-MNA as substrate) and the lysosomal exopeptidase dipeptidylpeptidase II (DAP II; Lys-Ala-2NA as substrate) were fluorometrically determined in the renal homogenate of normal and experimental (castration followed by a 14-day treatment with estradiol and testosterone) rats of both sexes. In addition, methodological investigations of the renal homogenate were performed in order to differentiate cath B from other proteinases. These showed that cath-B activity was highest at around pH 6, was strongly inhibited by 4-hydroxymercuribenzoate and leupeptin, and was activated by dithiothreitol. Trypsin-like activities were not demonstrable under the used incubation conditions. The animal experiments showed that renal cath-B activities (1) were significantly higher in females than in males, (2) increased significantly in males and decreased significantly in females after castration (no significant difference between both sexes), (3) decreased in female and male castrates after treatment with testosterone and increased strongly after treatment with estradiol, and (4) showed an activity pattern similar to that of DAP II. The results are discussed in relation to the sex-dependent and sex-hormone-dependent proteinuria of rats. It is suggested that there is a correlation between protein catabolism in the kidney and proteinuria, i.e. high lysosomal proteinase activities correspond with low proteinuria.