1. The effect of catecholamines on the time course and amplitude of contraction and on KCl-induced contractures has been studied in mammalian hearts.2. Marked and reproducible contractures could be obtained in mammalian ventricular trabeculae and papillary muscles after beta-adrenergic block with propanolol or if the hearts were depleted of their catecholamine stores by reserpine or by chemical denervation with 6-hydroxydopamine.3. In neonatal hearts with lower endogenous catecholamine stores and poorly developed sarcoplasmic reticulum KCl contractures are easily produced.4. Catecholamines potentiate twitch tension and relax the contracture tension under all of the above circumstances.5. The relaxant effect of catecholamines is present during the time course of a twitch. This increased relaxation rate as well as the shortening of the time-to-peak of tension is independent of the variation in the duration of the action potential.6. The shortened relaxation time is present when the action potential is shortened with anodal repolarization or prolonged with cathodal depolarization (voltage-clamp).7. The relaxant effect of catecholamines on the twitch is temperature and rate dependent. The effect is observed in the presence of high or low concentrations of calcium.8. The presence of catecholamines is necessary for full relaxation of mammalian heart muscle under high performance conditions or states of calcium overload.9. It is proposed that catecholamines exert their relaxant effect independent of their positive inotropic effect by stimulating the sequestering system (sarcoplasmic reticulum, mitochondria or sarcolemma) for calcium.