The dependence of the cathepsin B1 (EC 3.4.22.1) activity on pH of a medium with substrates N alpha-benzoyl-L-arginine amide (BAA) and N alpha-benzoyl-DL-arginine-paranitroanilide (BAPNA) was studied. Several maxima of the studied enzyme activity are established depending on pH in the grey and white matter of the cat cerebral hemispheres. The activity of cathepsin BI is different in functionally and morphologically different areas of brain (grey matter of cerebral hemispheres less than than cerebellum less than white matter of cerebral hemispheres less than medulla oblongata). A six-fold freezing and defrosting of homogenates of the brain structures under study causes an increase in the cathepsin B1 activity. It is shown that the cathepsin B1 activity in the brain grey matter grows in the presence of L-cysteine and is inhibited under the effect of pChMB, monoiodoacetic acid, Mn2+ and Cu2+.