Propranolol, diphenylhydantoin and tetrodotoxin showed inhibitory or removal effects on fibrillation induced in isolated rabbit atrial muscle strip. Depending upon methods of induction of fibrillation, effects of these drugs were different. The mode of their action seemed to be due to following electrical natures of each drug in its effective dose.Diphenylhydantoin decreased rate of rise and amplitude of the action potential of dog ventricular fibers and rabbit atrial fibers, which was also a cause of prolongation of conduction time. Duration of action potential was slightly prolonged in an effective low concentration. In contrast propranolol showed a relatively little effect on these items, but depressed ectopic spontaneous activity. Tetrodotoxin had both effects more markedly.