Doppler echocardiographic indices of diastolic function and systemic haemodynamics were studied in response to infusions of atrial natriuretic peptide (0.5, 1, 2, 5 pmol.kg-1.min-1) and placebo (0.9% (w/v) saline) in ten normal male subjects. Compared with placebo, atrial natriuretic peptide infusion produced a significant and dose-related reduction in the isovolumic relaxation time [(mean and 95% CI) -5.9 (-9.2 to -2.6) ms (P < 0.01) at 5 pg.kg-1 min-1] and a significant increase in the ratio between early and late transmitral peak velocities [0.46 (0.02 to 0.89) (P < 0.05) at 5 pg.kg-1 min-1]. No significant changes in heart rate, blood pressure or aortic stroke distance were observed with infusion of atrial natriuretic peptide compared with placebo. These data suggest that pathophysiological plasma concentrations of atrial natriuretic peptide improve diastolic function by increasing the rate of myocardial relaxation.