Elastase was found to be secreted in the pancreatic juice of the dog in its inactive, proelastase form in a concentration (control period) ranging between 400μg/ml to 500μg/ml. No active elastase was present. The pancreatic juice was found to contain an elastase inhibitor. Plasma contains elastase in a concentration found to range between 2.5μg/ml and 10μg/ml with a mean of 5.9 (SE ± 1.0). Plasma normally contains an elastase inhibitor found to be capable of inhibiting approximately 50% of the elastase added in vitro. Pancreozyminsecretin stimulation resulted in a decrease in concentration but an increase in total amount of elastase secreted in pancreatic juice, roughly paralleling the secretion of the other pancreatic enzymes measured. No change was detected in the levels of plasma elastase or in its plasma inhibitor.