Measurements of amylase activity and its isoenzyme pattern in the serum and urine were made in 53 cases of various diabetic type. The transition before and after treatment was investiga ted in some diabetics. The following results were obtained. 1) The serum amylase activity and its isoenzyme pattern were very similar between juvenile onset diabetes (JOD) and secondary diabetes mellitus due to chronic pancreatitis. 2) Pancreatic amylase in JOD was significantly lower than that in maturity onset diabetes (MOD)(p<0.O01). 3) The serum amylase activity was significantly increased after treatment of diabetes mellitus (p<0.001), but amylase clearance remained unchanged. The study on serum amylase isoenzyme indicated that pancreas-originating amylase was not increased in JOD, although both salivary and pancreas-originating amylase were increased in parallel in MOD. 4) In the case of diabetic ketoacidosis, the serum amylase activity was low before treatment. In some patients, the amylase activity was markedly raised during treatment with small intravenous insulin boluses. The increased amylase activity was due mainly to salivary type. These results suggest that the production of amylase may be strongly dependent on the control of the diabetic state.