The purpose of this study was to evaluate the cardiovascular, renal and endocrine effects of human atrial natriuretic factor (ANF), infused at a rate of 0.8 microgram/min (about 4 pmol/kg/min) for three hours in normal subjects and patients with essential hypertension. This infusion rate was chosen to obtain a range of plasma ANF levels which can be generated by physiological manoeuvres and to reduce the likelihood of hypotension. Five patients and six healthy volunteers participated in the study. The infusion had to be prematurely discontinued in one patient and in one control because of hypotension with relative bradycardia. Blood pressure otherwise remained unchanged during infusion whereas heart rate rose transiently. Plasma ANF levels increased similarly during infusion from 8.9 +/- 2.6 to 23.9 +/- 6.4 pmol/l in patients and from 3.7 +/- 0.7 to 25.4 +/- 6.9 pmol/l in the controls, remained stable during the infusion, and decreased similarly in both groups after the infusion, with a half-life of 7 min. Plasma guanosine cyclic phosphate (cGMP) was augmented by about four-fold in both groups. In both groups, plasma aldosterone levels fell whereas plasma noradrenaline increased. The diuretic effect of ANF was similar in both controls and patients (1354 +/- 161 vs 1542 +/- 116 ml/3 hrs respectively), whereas its natriuretic effect was exaggerated in hypertensive patients (90 +/- 11 vs 62 +/- 9 mmol/3 hrs, P less than 0.05). In conclusion, this low infusion rate of ANF produced similar changes in plasma ANF, cGMP, aldosterone and noradrenaline levels but patients with mild essential hypertension demonstrated an exaggerated diuretic and natriuretic response to ANF infusion.