Pancreatic enzyme secretion in rats anesthesized by pentobarbital was stimulated by intravenous perfusion of the hormone pancreozymin, as indicated by a decreased amylase level in the pancreas and by specific, fine structural changes observed in an electron microscope . Rates of protein synthesis were determined by pulse labeling . Amylase, total protein, and valine were purified from pancreas and counted . Pancreozymin promotes an 8 to 10 times increase in the rate of biosynthesis of pancreatic enzymes, as compared to rats similarly anesthesized but without hormone . This stimulation effect is obtained very rapidly (2 hr) and is not inhibited by actinomycin D. Secretin alone has no effect, whereas pentobarbital is inhibitory .