Specific angiotensin II (ANG II) binding on human platelets in man was measured to examine ANG II receptor regulation in man. Eight normal volunteers received either a high, or a low sodium diet, or a low sodium diet together with the ANG I converting enzyme inhibitor enalapril. The following parameters were determined at the end of each study period: ANG II platelet receptor binding capacity (Bmax) and dissociation constant (KD); plasma-ANG II, plasma renin, plasma-aldosterone, plasma-atrial natriuretic peptide; and blood pressure (BP) increase to graded doses of i.v. given ANG II. Plasma ANG II (6 +/- 0.5, 21 +/- 4, 8 +/- 1 fmol/ml) and Bmax (14 +/- 2, 5 +/- 1, 10 +/- 2 sites/cell) changed in a reciprocal fashion on a high, low or low sodium diet with enalapril respectively. Plasma concentration of atrial natriuretic peptide was unchanged on a low sodium intake and consistently decreased when enalapril was added to the low sodium regimen. BP increase to 4 ng ANG II/kg/min was 27 +/- 3, 10 +/- 1, and 24 +/- 5 mmHg during the three study periods respectively. The data suggest that ANG II regulates maximal capacity of platelet ANG II binding. Therefore, regulation of platelet ANG II binding sites goes in parallel with, and may serve as index for, ANG II receptors on vascular smooth muscle, but ANG II receptor regulation is apparently different in adrenal cells.