Mongrel dogs were fed, from weaning to 9 months of age, on one of two diets that differed only in the type of fat content (virgin olive oil or sunflower oil) to compare the composition of exocrine pancreatic secretion in the basal period and in response to food. In resting pancreatic flow, electrolytes and the specific activities of amylase, lipase and chymotrypsin were similar in both experimental groups. However, lipase and amylase outputs, and amylase and protein concentrations were significantly higher in the group fed on the diet rich in sunflower oil. Food intake was not followed by any change in flow-rate or electrolyte or protein content in the group given the diet rich in olive oil. Amylase activity and output were also lower in this group, as was lipase output, whereas activity and specific activity of chymotrypsin were lower in dogs fed on the diet containing sunflower oil. The differences traceable to the composition of the two types of dietary fat supplied may be related to the balance between factors that stimulate and inhibit pancreatic secretion.