The aim of the present study was to determine whether left atrial size--a likely indicator of atrial stretching--correlates with the plasma concentration of atrial natriuretic peptide and whether this relation is different in patients in sinus rhythm and in those with atrial fibrillation. Arterial plasma concentrations of immunoreactive atrial natriuretic peptide (ir-ANP), adrenaline, noradrenaline, aldosterone, and vasopressin were measured in 13 patients in sinus rhythm without apparent heart failure and in 13 patients in atrial fibrillation. The two groups were matched for left atrial diameter and the ratio of the left atrial diameter to the diameter of the aortic root (assessed by echocardiography). There were no significant differences in age, heart rate, blood pressure, or left ventricular end diastolic diameter between the two groups. Left atrial diameters varied from 33 to 60 mm. The mean (SD) plasma concentration of ir-ANP was significantly higher (35 (21) pmol/l) in the patients with atrial fibrillation than in those in sinus rhythm (12 (11) pmol/l). The concentration of plasma aldosterone was also higher in patients with atrial fibrillation (831 (366) v 523 (211) pmol/l). Concentrations of adrenaline, noradrenaline, and vasopressin were similar in both groups. None of the hormone concentrations correlated with left atrial dimensions. These results indicate that plasma concentrations of ir-ANP and aldosterone are highly sensitive indicators of changes in haemodynamic function during atrial fibrillation. They also underscore the difficulties of correlating echocardiographic assessment of patients with plasma concentrations of a vasoactive hormone.