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The stimulatory effects of isoproterenol and secretin on external pancreatic secretion were compared in the rat. 1. In acute fistula, pylorus ligation, atropine, glucagon did not change either of the stimulated secretions. Propranolol inhibited isoproterenol-induced secretion and did not change secretin induced stimulation. Theophylline alone displayed a large hydrelatic stimulatory effect, without increasing protein excretion; the effect of theophylline was additive with the effects of isoproterenol or secretin but no evidence was found of potentiation or prolongation of action. Isoproterenol did not increase pancreatic blood flow and induced no variations in the plasma levels of immunoreactive secretin. Combining various doses of isoproterenol and secretin did not allow to reach secretory levels greater than the maximal response to secretin. 2. In conscious rats chronic fistulae, isoproterenol and secretin had a distinct effect.