Isolated acinar cells from rat pancreas responded well to hormonal treatment. Both secretin (synthetic and highly purified from porcine origin) and CCK-octapeptide stimulated amylase secretion in these cells. The response in both cases was very rapid. A maximal output of enzyme was reached within 5–lOmin after the addition of hormones. The concentration producing maximal output for synthetic secretin (Schwarz/Mann) was 5 X 10-8M, synthetic secretin (Squibb), 10-5M, for purified porcine secretin, 10-5M, and for CCK-octapeptide was 5 X 10-10M. Secretin (2-fold at optimal concentration) was found to be less efficient compared to CCK-octapeptide (5-fold at optimal concentration) in stimulating amylase release. A combination of secretin and CCK-octapeptide had a synergistic action in stimulating enzyme release by the acinar cells. In addition, pretreatment of acinar cells with secretin potentiated the secretory response of the treated cells to CCK-octapeptide. To a lesser extent pretreatment with CCK-octapeptide also increased the effect of secretin in stimulating enzyme secretio