Cholecystokinin (CCK) is a major peptide hormone in the gut and a major peptide transmitter in the brain. Its synthesis requires endoproteolytic cleavage of proCCK at several mono- and dibasic sites by prohormone convertases (PCs). Of these, PC1 and PC2 are expressed in cerebral neurons and intestinal endocrine cells. Characteristically, however, the processing of proCCK varies markedly between the brain and the gut. In neurons, CCK-8 is always the predominating form, whereas the endocrine gut cells (I-cells) contain a mixture of small and larger CCK-peptides of which CCK-33 or CCK-22 often predominate. The role of PC1 and PC2 in the processing of proCCK have now been examined by measuring the concentrations of prohormone, processing intermediates and amidated end-products in jejunal and cerebral extracts of PC1 and PC2 deficient mice and corresponding wild type controls. The PC1 null mice revealed a pattern opposite to that of the PC2 null mice, in whom only the cerebral processing of proCCK was affected. Thus PC1 knockouts reveal a severe block in the processing of intestinal proCCK. Accordingly, the intestinal concentration of proCCK was many fold increased, and also the concentrations of different processing intermediates were raised; but the concentration of bioactive, alpha-amidated and O-sulfated CCK was reduced to a few percent of normal. The only bioactive CCK peptide in the gut of PC1 deficient mice was CCK-22, and it was present only in trace amounts. The cerebral processing of proCCK was, however, not at all affected by the lack of PC1 - in sharp contrast to the effect of PC2. The results show that the tissue-specific processing of proCCK to a large extent can be explained by prohormone convertases, as PC1 plays a decisive role in the maturation of hormonal CCK in the gut, whereas PC2 governs the processing of brain proCCK.