1. Nine patients with compensated heart failure were infused with synthetic arginine vasopressin at a rate of 0.1 m-units min-1 kg-1 for 60 min to increase their plasma arginine vasopressin concentration. Synthetic human atrial natriuretic factor (3 pmol min-1 kg-1) or placebo was co-infused with the arginine vasopressin in random order in a single-blind cross-over design. 2. The resultant plasma concentrations of arginine vasopressin and atrial natriuretic factor fell to within the upper range observed in congestive heart failure. Compared with the infusion of arginine vasopressin alone, atrial natriuretic factor co-infusion enhanced both the urine flow rate and the sodium excretion rate (both P less than 0.05) without significant haemodynamic and hormonal effects. 3. Systematic blood pressure was elevated by arginine vasopressin infusion (P less than 0.05) without any change in heart rate. Co-infusion of atrial natriuretic factor did not affect these haemodynamic parameters. 4. These results suggest that an increased release of atrial natriuretic factor maintains water and sodium excretion in the presence of arginine vasopressin-induced renal modulations, and that the pressor effect of arginine vasopressin is not antagonized by the increased plasma level of atrial natriuretic factor in patients with congestive heart failure.