The effect of dietary fat on levels of lipase and other enzymes in rat pancreas has been studied. It was possible to raise levels of lipase in animals by supplementing their commercial chow diet with added fat or by raising the level of fat in semipurified diets from 4% to 22%. Pancreatic amylase levels decreased in rats fed the high fat diets, whereas levels of chymotrypsinogen and trypsinogen were unaffected. The type of carbohydrate in the semipurified diets made no difference. Thus, the levels of enzymes in rats fed dextrose-containing diets or cornstarch-containing diets were similar. On the basis of the present data, and results of others, it would appear that levels of pancreatic lipase are increased when the fat content of the diet is raised from about 5% to 15-22%, but that little or no additional increase in lipase levels can be attained by any further increase in the amount of dietary fat.