SUMMARY A renin-like enzyme and add protease (cathepsin) from whole and saline-perfused dog brains were separated by CM-cellnlose cbromatography with a linear NaCl gradient. Plasma renin and cathepsin were also separated using the same system. During the separation steps (in all the above cases) the specific activity of the brain renin-like enzyme was increased, while the specific activity of the brain cathepsin was decreased. Approximately a 70-fold increase in the specific activity of brain renln-like enzyme and a sixfold decrease in brain cathepsin specific activity was obtained from saline-perfused brain. The separation made it possible to study the pH optimum of the brain renin-like enzyme and acid protease. The brain renin-like enzyme showed optimal activity in the range of pH 6-7. Immunologically, the renin-like enzyme was distinctly different from dog kidney renin.