Methoxyflurane reduced the total developed tensions of isolated guinea pig atria and isolated cat papillary muscle but did not change the rate of spontaneous beating of isolated atria. The staircase phenomenon and postextrasystolic potentiation appeared not to be affected when the increases in developed tension were expressed as percentages of the preceding values. These effects are similar to those reported for halothane. Unlike halothane, methoxyflurane did not produce significant changes in the increases of developed tension induced in papillary muscle by high-voltage electrical stimulation or tyramine or in the effective refractory period of papillary muscle.