It is well-known that the prevalence of hypertension progressively increases with body weight. Since the major physiological activities of atrial natriuretic factor (ANF) are its natriuretic, diuretic and vasodilatory effects, the aim of the present study was to investigate the ANF basal plasma levels and platelet receptor number in 15 young normotensive obese (O) and 12 age-matched normal weight healthy (C) women. ANF effectiveness was also studied in eight of the obese and seven of the control women, after an intravenous bolus of the hormone (human ANF (99-126): 0.5 mg/kg body weight). Results are expressed as means+s.d. Basal ANF plasma levels were similar between obese (18.2 +/- 9.7 pg/ml) and control women (12.3 +/- 7.7 pg/ml), whereas obese patients showed an increase density of platelet ANF-binding sites (clearance receptors) (C: 28.7 +/- 33.5 fmol/10(9) cells; O: 39.6 +/- 4.6 fmol/10(9) cells; P < 0.001), without apparent differences in receptor affinity (Kd) (C: 6.0 +/- 3.0 pM; O: 7.0 +/- 2.0 pM). The biological response to ANF, evaluated by changes of mean blood pressure levels (C: 5 +/- 1 mmHg; O: 1 +/- 1 mmHg; P < 0.001) and the percentage increase of diuresis (C: 159 +/- 52%; O: 81 +/- 62%; P < 0.01) and natriuresis (C: 205 +/- 129%; O: 99 +/- 41%; P < 0.05) was significantly reduced in obese patients. The percentage increase in sodium excretion was inversely related to the basal insulin concentrations in the obese group (r = 0.64, P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)