Under different experimental conditions, endogenous noradrenaline and contractile force of the isolated rat heart were measured concomitantly. Neither substrate-depleted nor hypoxic hearts, which failed to generate contractility, showed a proportional decline in noradrenaline concentration. The catecholamine stores decreased but the contractile force was maintained in hearts perfused with medium containing glucose for 2 hr. In hearts from animals treated with reserpine, catecholamine stores were markedly reduced and contractility increased while, in hearts from animals treated with nialamide, catecholamine stores were increased without any change in contractility. Essentially a good correlation was noted between results concerning catecholamine stores measured by chemical and histochemical techniques. The data do not suggest any relation between endogenous noradrenaline stores and myocardial contractility.