The present study characterizes the hemodynamic and neurohumoral responses to moderate treadmill exercise in conscious dogs with pacing-induced heart failure. Seven dogs were instrumented with a left ventricular micromanometer, ultrasonic crystals for the measurement of systolic wall thickening, left atrial and aortic catheters for the injection of colored microspheres and reference withdrawal, respectively, and ventricular pacing leads with a subcutaneous pacemaker. Dogs were run on a treadmill at a speed of 5 km/h. After control studies, heart failure was induced by rapid left ventricular pacing at 250 beats/min for (mean +/- SD) 23 +/- 6 days. In the control state, cardiac output was increased from 4.5 +/- 1.5 to 7.9 +/- 1.4 l/min (P < 0.05 vs. rest). With heart failure, cardiac output was decreased to 2.5 +/- 0.5 l/min at rest (P < 0.05 vs. control state) and was only 3.0 +/- 0.3 l/min during exercise (P < 0.05 vs. control state; not significant vs. rest). Myocardial and, more so, skeletal muscle blood flows at rest were reduced in heart failure; their increases with exercise were attenuated. An increase in renal blood flow during exercise in the control state was no longer seen in heart failure. Increases in plasma catecholamines and lactate during exercise were more pronounced in heart failure. In conclusion, in heart failure, the increase in cardiac output during exercise was largely attenuated. Increased catecholamine levels may have contributed to splanchnic vasoconstriction and preferential distribution of cardiac output into the working skeletal muscle.