Nine patients with chronic obstructive lung disease underwent right heart catheterization. Mean pulmonary arterial pressure was elevated, but none of the patients had clinical signs of cardiac failure. Mean arterial oxygen saturation and carbon dioxide tension were 89% and 47 mmHg, respectively. Plasma noradrenaline (NA) concentration averaged 0.51 ng/ml and was inversely correlated to arterial oxygen saturation and mixed venous oxygen saturation, and positively correlated to arterial carbon dioxide tension and mean pulmonary arterial pressure. Oxygen inhalation did not change plasma NA significantly. Eight months later, plasma NA was increased (mean 0.89 ng/ml), as compared to the first determination (P < 0.02), and to a group of normal subjects (mean 0.26 ng/ml, P < 0.005). Inverse correlation to arterial oxygen saturation and direct correlation to arterial carbon dioxide tension were still present. Plasma adrenaline concentration was normal. The results point to enhanced sympathetic nervous activity in patients with chronic obstructive lung disease, probably caused by the deranged blood gases. The pulmonary haemodynamic changes and increased pulse rate may, at least partly, be due to enhanced sympathetic nervous activity.