Static measurements of plasma neurohormones at rest may not be adequate to detect alterations in cardiovascular control mechanisms in congestive heart failure (CHF). Therefore, it is of interest to study neurohormonal activation during different physiological conditions. Plasma neurohormones were measured in 54 patients on diuretic therapy for mild or moderate CHF. Samples were taken at rest and immediately after maximal bicycle exercise, before and after 12 weeks of treatment with ramipril or placebo. There was a strong correlation between the plasma levels of each hormone before and after exercise. An inverse correlation existed at baseline between exercise duration and angiotensin II levels after maximal exercise (r = -0.30, P = 0.03), but not at rest. Plasma levels of angiotensin II, aldosterone, atrial natriuretic peptide (ANP) and noradrenaline were increased after maximal exercise compared to rest. Plasma angiotensin converting enzyme activity and ANP were reduced by ramipril compared to placebo, both at rest and after exercise, but levels of angiotensin II, aldosterone and nordrenaline were not significantly affected. Thus, exercise consistently activates neurohormonal systems in patients with CHF. Patients with the lowest exercise duration had the highest angiotensin II levels after exercise. Measurements of plasma neurohormones after maximal exercise provide limited additional value to measurements at rest.