In vitro experiments were carried out to examine the enzymatic activity in various organ homogenates of the dog, which inactivates the biological activity of the synthetic cholecystokinin-octapeptide (CCK-OP). The highest splitting activity was found in the renal cortex; substantially lower activities were registered in the lung, pancreas and small intestine. It seems interesting that neither the gallbladder nor the saliva and the serum contained measurable amount of the CCK-OP splitting activity. An effort was made to characterize the CCK-OP inactivating principle found in the renal cortex. It was ascertained that the CCK-OP was inactivated by a peptidase which is heat sensitive, has a pH optimum of 7,4 and could be inhibited by chelating agents (EDTA) and epsilon-aminocaproic acid. The fact that most of the enzyme function is associated with the sediment obtained at 12 000 g speaks in favour of its mitochondrial origin.