The contractile function of the left ventricle has been defined within the framework of the pressure-volume relationship. We employed a conductance catheter, together with a high-fidelity micromanometer, to obtain accurate pressure and volume data continuously on a beat-to-beat basis in conscious dogs. Reproducibility of conductance volumetry was proven by repeated in situ measurements of left ventricular volume on separate days in the same dog. Heart failure, produced by rapid ventricular pacing, was characterized by impaired systolic shortening, depressed contractility indices and incomplete left ventricular relaxation. The magnitude of the cardiotonic effects of dobutamine was significantly attenuated after development of heart failure. Dobutamine improved left ventricular early relaxation but did not affect chamber distensibility. In heart failure, the load sensitivity of relaxation was enhanced and the force-frequency response attenuated. This may elucidate the mechanisms of early diastolic dysfunction and the deleterious effect of an increase in heart rate in the failing heart. Thus, the conductance catheter provided a reliable and simple method of obtaining left ventricular volume continuously in conscious dogs.