The effect of a synthetic analogue of atrial natriuretic peptide (Ileu-ANP) on haemodynamic, hormonal, and electrolyte excretion indices was studied in 7 patients with chronic congestive heart failure. Patients received in random order placebo or Ileu-ANP infusions (5 micrograms/min) for 4 h on 2 separate occasions, at least 1 week apart. Compared with placebo, Ileu-ANP caused significant reductions in mean systemic arterial pressure, mean pulmonary artery pressure, pulmonary diastolic pressure, and right atrial pressure. These changes were sustained for at least 2 h after infusion. Cardiac output increased from 6.2 to 7.4 l/min at 60 min, then returned to pre-infusion levels. Despite considerable falls in systemic pressure there was no significant increase in heart rate or plasma noradrenaline. With Ileu-ANP infusion, plasma renin activity, angiotensin, arginine vasopressin, aldosterone, and cortisol values were not significantly different from placebo values. Plasma cortisol and aldosterone increased after stopping Ileu-ANP. Neither urine volume nor sodium excretion rate was significantly increased by Ileu-ANP.