The secretion of protein by the acinar cells of the pancreas occurs in two steps: the synthesis of the protein from amino acids and its migration through the cell to the acinar lumen. Since very soon (one minute) after the injection of leucine-H3 the labeled protein is found mainly in the ergastoplasm of the basal half of the cell, protein synthesis is believed to occur in this location. At later intervals the labeled protein is found in the Golgi complex (10 and 30 minutes), and then in the zymogen granules of the apical half of the cell (60 minutes). The newly synthesized protein thus migrates from the ergastoplasm to the Golgi complex, and from there as zymogen granules through the apical half of the cell prior to being released in the acinar lumen.