A singel subcutaneous dose (1 mg/kg) of isoproterenol (ISO) increases the net calcium-45 uptake into the ventricular myocardium of normal rats by a factor of 4 within 6 hours. When the same dose of ISO was administered to rats with aortic constriction the rise in labelled Ca uptake by the hypertrophied myocardium of the left ventricle was greatly reduced. A decrease in ISO-induced Ca uptake also occurred in the non-hypertrophied right ventricular myocardium. Consequently, the stimulatory effect of ISO on the Ca-dependent high energy phosphate consumption was considerably diminished. A similar restriction of ISO-induced radiocalcium uptake and ATP utilization occurred in hypertrophied hearts of chronically ISO-treated rats or when the rats were exposed to other forms of experimental stress such as bleeding or cold. Although the ISO-induced cardiomegaly was reversed within 7 days, the insensitivity to beta-adrenergic stimulation persisted for more than 3 weeks. The results indicate that the decrease in responsiveness to beta-adrenergic agents is not due to the hypertrophy itself but must be considered to be an independent phenomenon - possibly resulting from adaptation to chronic beta-adrenergic overstimulation.