We have studied the capacity of different canine tissue homogenates to destroy cholecystokinin in vitro. Tissues from the kidney cortex, lung, pancreas, and small bowel contained significant cholecystokinin-destroying activity. Only small amounts of activity were found in the liver, and no activity was detected in the kidney medulla, the gastric antrum, gallbladder, spleen, or in serum or plasma. The kidney cortex was the richest source of activity. The cholecystokinin-destroying enzyme isolated from the kidney cortex was heat-labile, non-dialyzable, and trypsin-resistant, with an optimum pH between 7 and 7.4. The enzyme was inhibited by chelating agents and by phenylalanine amide, although it was little affected by phenylalanine itself or proteinase inhibitors. The enzyme required divalent cation cofactors for its activity. After inhibition by EDTA, the enzyme could be reactivated completely with Mn2+, but not with Ca2+ or Mg2+ alone. The cholecystokinin-destroying enzyme was bound strongly to membranes, and during differental centrifugation, was sedimented with mitochondrial fractions of kidney cortex. The supernatant fraction of the solubilized enzyme obtained at 105,000 X g had a molecular weight of about 480,000 dalton. Since the enzyme could be inhibited by phenylalanine-amide, it would appear to act at the C-terminal end of the cholecystokinin molecule.