1. Skeletal muscle metabolism in chronic cardiac failure may be influenced by the many circulatory and neurohumoral adaptations in the condition. We investigated aerobic metabolism during exercise using indirect calorimetry in 15 patients with chronic cardiac failure and in 14 control subjects. Subjects exercised on a treadmill for 20 min at a steady-state submaximal workload. 2. Venous lactate levels were relatively constant throughout the exercise, although they were slightly greater in the patients with chronic cardiac failure than in the control subjects. The respiratory exchange ratio was lower in patients with chronic cardiac failure (0.777 +/- 0.021 vs 0.833 +/- 0.037, means +/- SD; P less than 0.0002, Mann-Whitney U-test). Relative fat utilization, expressed as a percentage of total energy expenditure, was therefore greater in patients with chronic cardiac failure (71.8 +/- 7.0 vs 54.1 +/- 12.3%, P less than 0.0005) and this was mirrored in a lower carbohydrate utilization (24.7 +/- 6.5 vs 43.3 +/- 12.1%, P less than 0.0002). Levels of free fatty acids, glycerol and noradrenaline were greater in patients with chronic cardiac failure. 3. We conclude that there is an increased reliance on fat, as opposed to carbohydrate, metabolism during exercise in chronic cardiac failure, and that this may relate to the elevated catecholamine and free fatty acid levels present. This may be a compensatory mechanism to preserve muscle glycogen stores, but as fat utilization is less efficient in terms of oxygen consumed, this shift may further impair exercise capacity.