In chronically prepared dogs the maximum acceleration of ejected left ventricular blood (Qmax) and the tension—-time index (TTI) were used as correlates of left ventricular myocardial contractility and integrated wall tension, respectively. These variables were measured in the conscious animal and at end-expired air concentrations of 1, 1.5, and 2 per cent halothane and 20, 25, 30, and 35 per cent cyclopropane. Qmaxwas depressed 65 per cent by 1.5 per cent halothane and 33 per cent by 30 per cent cyclopropane. TTI was decreased 29 per cent by halothane and increased 47 per cent by cyclopropane. Cyclopropane decreased myocardial contractility more than has been previously reported in studies in which this variable was evaluated from the relationship between ventricular stroke work and filling pressure. The divergent effects of halothane and cyclopropane on TTI suggest that left ventricular oxygen consumption may be greater during the administration of cyclopropane.