The activity of the sympathetic nervous system is increased at rest in patients with congestive heart failure. To determine whether this augmentation is carried over during dynamic upright exercise, 14 patients with congestive heart failure were stressed maximally during upright bicycle ergometry. Plasma norepinephrine and epinephrine levels were measured in the basal upright (sitting) posture before and during maximal exercise. The results were compared with those in six healthy control subjects before and during maximal exercise. Plasma norepinephrine increased during exercise from a mean (± standard error of the mean) of 650 ± 95 to 1,721 ± pg/ml in the group with heart failure. This increase was significantly less (p < 0.001) than that in the control group (from 318 ± 36 to 3,230 ± 418 pg/ml). However, for equivalent levels of total body oxygen consumption (V̇O2), the group with heart failure had higher levels of plasma norepinephrine than the control group. Plasma epinephrine was similar in the two groups in the basal upright position (92 ±18 and 92 ± 26 pg/ml), but it increased more during exercise in the normal subjects (743 ± 210 pg/ml) than in the group with heart failure (167 ± 67 pg/ml) (p < 0.001). The percent increase in norepinephrine correlated with the percent change in V̇O2 in the group with heart failure (r = 0.62, p < 0.02), but the percent change in epinephrine did not. There is, therefore, a disturbance in the sympathetic nervous system during exercise in patients with congestive heart failure. Although norepinephrine increases in such patients to a greater extent than in normal subjects at lower levels of exercise, the extremely high levels of norepinephrine and epinephrine generated by normal subjects during maximal upright exercise do not occur in patients with heart failure.