In the 1950's, the specific cleavages of the peptide bonds occurring in bovine cationic chymotrypsinogen and trypsinogen were among the first examples of limited proteolyses. According to the split bond(s), the precursor is transformed into enzyme or different forms of zymogen or again into inert protein. The conversion of trypsinogen into trypsin triggers the activations of all the other enzyme precursors of pancreatic juice. In the pancreas, several factors oppose trypsinogen autoactivation, whereas, in the duodenum, all the conditions are favorable for trypsinogen activation by enteropeptidase.