Plasma concentrations of the atrial natriuretic peptide (ANP) were measured during cardiac catheterization in 289 patients with heart disease. It was elevated in all types of cardiac disease investigated, irrespective of the nature and duration of the disease. Close relationships were observed between the elevated ANP concentrations and the increased right and/or left atrial pressure. Further increments in ANP concentration were measured during exercise in direct response to the rise in mean pulmonary artery pressure. Thus, ANP concentrations may be regarded as a non-invasive marker of the haemodynamic burden in cardiac disease.