After a single-blind, randomized, cross-over protocol using decaffeinated coffee in a control experiment, the effect of an oral 250-mg caffeine dose on plasma immunoreactive atrial natriuretic peptide (ANF) was assessed in eight healthy students who had been on a methylxanthine-free diet for 1 week. One to 2 h after caffeine ingestion, both systolic blood pressure (SBP) and diastolic BP (DBP) increased by 12 mm Hg while heart rate (HR) also tended to increase. An increase in diuresis and in urinary sodium, potassium, and osmol excretion was observed within 1 h. Decaffeinated coffee induced no change in any of these parameters. Plasma epinephrine (EPI) increased gradually from 16.6 +/- 3.2 pg/ml (mean +/- SEM) to 45.1 +/- 7.9 pg/ml within 2 h after caffeine ingestion, but did not change after decaffeinated coffee (p less than 0.001). Plasma norepinephrine (NE), renin activity (PRA), aldosterone, and vasopressin remained unchanged. Plasma ANF was measured by radioimmunoassay (RIA) using an extremely sensitive antiserum (Kd = 10(-12) M) after rapid and virtually complete (90-103%) extraction from plasma. In 0.2 ml plasma, the theoretical detection limit is 1.1 fmol/ml. Normal plasma ANF concentrations in supine subjects were 17.9 +/- 8.1 fmol/ml (mean +/- SD) and 11.0 +/- 3.3 fmol/ml in subjects in the upright position. Plasma ANF levels were not affected by coffee drinking. In conclusion, by using a new and sensitive assay for plasma ANF, we did not find that caffeine-induced diuresis is mediated by ANF.