Studies are presented dealing with the effects on exocrine and endocrine rat pancreas of repeated subcutaneous injections for 10 days of equivalent doses (Ivy dog units) of commercial (10% pure) cholecystokinin-pancreozymin (CCK-PZ) and of the synthetic C-terminal octapeptide of CCK-PZ. Both rats treated with the commercial CCK-PZ and the octapeptide had an increase wet weight of the pancreas. Pancreatic concentrations of amylase, lipase, and trypsinogen increased in a parallel fashion after both kinds of hormonal treatment. Rats given the 10-day treatment with the commercial CCK-PZ preparation displayed a lower insulin response following an intravenous glucose load. The glucose tolerance, however, was slightly improved. The octapeptide treatment induced no alterations of the insulin or glucose levels after similar glucose loads. The insulin content and the concentration of protein in pancreatic tissue or glycogen in liver and muscle tissue were unaffected by both ways of treatment. Likewise no difference was found on glucose utilization in muscle tissue in vitro. It is concluded that equipotent doses of the synthetic octapeptide and the 10% pure CCK-PZ preparation induce a similar and parallel increase of the concentration of all three exocrine pancreatic enzymes. The inhibitory effect on glucose-induced insulin release exerted by the 10-day treatment with commercial CCK-PZ but not by the octapeptide is conceivably due to peptide impurities in the commercial CCK-PZ and/or to parts of the CCK-Pz molecule other than the C-terminal octapeptide.