A study was made of the changés produced in action potentials of frog ventricular cells by variation in ion concentrations, by administration of NaN3, 2:4 DNP, NaCN, NaIA and N2which shortened the potentials. The exposure of the perfused tissue to ionized acridines, barbiturates and desoxyglucose caused a prolongation of the action potentials. These actions as obtained were reversible. The evidence from the time-course and differential effects of inhibitors suggests that an active ion transport, modulated by KO+may be important in maintaining the plateau phase of the action potential.