An investigation of changes in the Mg2+ -dependent, Na+ -K+ -stimulated sarcolemmal ATPase and of intracellular electrolytes in the left failing heart due to pressure overload (aortic banding) was carried out in dogs. There was no change in the sarcolemmal Mg2+ -ATPase of the left or right ventricle for the whole duration (3 to 9 months) of left ventricular pressure overload. In the early phase (3 months) of aortic banding, when there was no haemodynamic evidence of left ventricular failure, there was also no significant change in the sarcolemmal Na+ -K+ -ATPase, extracellular space, or intra- and extracellular electrolytes. However, during 6 to 9 months of aortic binding when there was haemodynamic evidence of left ventricular failure (increased end-diastolic pressure, decreased cardiac index and (dP/dt)/IIP, enlarged heart), there was also a marked increase in the left ventricular sarcolemmal Na+ -K+ -ATPase and intracellular K+; and a decrease in the intracellular Na+ and Ca2+. The extracellular space in the left ventricle also increased significantly. Unlike the left ventricle, the right ventricle did not show any evidence of failure, not did it show any change in the sarcolemmal Na+ -K+ -ATPase and intracellular electrolytes during any period of aortic banding. These results suggest that the decrease in the myocardial contractility in failing heart due to pressure overload might be associated with a decrease in the intracellular Ca2+ as a result of an increase in the sarcolemmal Na+ -K+ -ATPase.