1. Insulin (1 mu./ml.) potentiated the release of amylase from isolated pancreas of rats perfused with erythrocyte-containing medium and stimulated by 0-5 m-u. pancreozymin (Pz)/ml., whereas the same concentration of insulin failed to potentiate the response evoked by or 200 m-u. PZ/ml. 2. Intracellular measurement of membrane potentials from the acinar cells of the same preparations showed that insulin (1 mu./ml.) simultaneously potentiates the hyperpolarization and amylase release in response to 0-5 m-u. PZ/ml. 3. These effects of insulin on the PZ-induced responses were inhibited by ouabain (3 X 10(-5) M). 4. The results suggest that insulin, in the concentration studied, has a potentiating action on the activity of an electrogenic Na pump, which maintains the hyperpolarization and amylase release during continuous stimulation with Pz. 5. Insulin in also potentiated the Pz-induced amylase release in the rat pancreata in situ after vagotomy and ligature of the pyloric region.