Potassium canrenoate (KCR) is widely used in cardiac patients as an aldosterone antagonist, antiarrhythmic and diuretic drug. According to experimental and clinical studies it can also elicit an inotropic action. It is not clear, however, whether this inotropic activity occurs in the absence of any treatment or after the myocardial contractility has already been improved with digitalis. In order to evaluate a possible interaction of this drug with digitalis we administered KCR intravenously to 41 anaesthetized dogs either untreated or treated with digitalis, in which aortic and left ventricular pressures were recorded and myocardial contractility was evaluated by calculating in real time the first derivative of ventricular pressure (Formula: see text) max and two other contractility indexes (Formula: see text) max and V.max. The results obtained showed that KCR given at doses of 10, 20 and 30 mg/kg i.v. did not elicit any inotropic effect in dogs not previously digitalized. 100 mg/kg i.v. first depressed cardiac contractility and then increased it. After cardiac performance had been improved by digitalis, KCR further increased all contractility indexes significantly. These results could explain previous observations that no inotropic effect was observed in human subjects not treated with digitalis after treatment with KCR.