Chronic obstructive pulmonary disease (COPD) is associated with right heart failure and salt and water retention. The possible roles of haemodynamically active hormones in the early stages of COPD have not previously been described. Adrenaline, noradrenaline, renin activity, aldosterone, vasopressin, cortisol, growth hormone, prolactin and atrial natriuretic peptide (ANP) were measured during right heart catheterization in mixed venous blood and in a peripheral artery, in the supine and standing position, in two groups of patients with COPD: Group A with arterial oxygen tension (Pa,O2) < 8.0 kPa (60 mmHg) and Group B with Pa,O2 > 8.0 kPa (60 mmHg). A group of 15 control subjects was studied to obtain control hormonal measurements with a venous blood sample only. Haemodynamic and blood gas values and hormone levels were measured in the supine and standing positions to record changes in the various parameters in COPD patients, and the relationship between pulmonary haemodynamics and hormone levels. No differences were found in hormonal samples between peripheral artery and mixed venous blood. In comparison with the control group, both groups of COPD patients showed a significant reduction in cortisol (p < 0.0001) and in vasopressin (p < 0.005), and an increase in ANP (p < 0.05) and growth hormone (p < 0.05). A marked, but not significant, increase in renin activity, and aldosterone was also found. After standing the increment of adrenaline was significantly higher in COPD patients (p < 0.02). A significant inverse relationship was recorded between forced expiratory volume in one second (FEV1) and noradrenaline (p < 0.02). There is a complex hormonal response even in the early phase of chronic obstructive pulmonary disease. An increase of plasma levels of atrial natriuretic peptide appears to be the earliest neuroendocrine response in these patients.