In 12 dogs with acute experimental pancreatitis (AEP) and 6 control animals the "free", "latent" and "total" activity of acid phosphatase, beta-glucuronidase and cathepsins in whole homogenates of the pancreas, in a lysosomal-enriched subfraction and the supernatant of pancreatic tissue was estimated. AEP was induced by injection of bile salts and thrombin solution into the pancreatic duct. In 6 dogs the protection with heparin (1.5 mg/kg/body weight) immediately after producing AEP was applied. In AEP without any protection the free activity of hydrolases in the whole homogenate (80--90%) and in the lysosomal enriched subfraction (75--90%) was higher than in the controls (60--70% and 55--75% respectively), suggesting an augmented lysosomal fragility during the course of AEP. Heparin depressed the free activity of hydrolases to 60--80% in whole homogenates, and 64--75% in the lysosomal enriched subfraction. The release of cathepsins during incubation of the lysosomal-enriched subfraction in acidic medium was lower in the group with heparin treatment. The data obtained suggest the stabilising effect of heparin on the lysosomes of the pancreas during acute experimental pancreatitis in dogs.