The significance of atrial natriuretic peptide (ANP) was investigated in the maintenance of the fluid volume in hypovolemia associated with dumping syndrome following gastric resection. The study was performed on 10 patients who had undergone a Billroth II procedure. Ten age- and sex-matched patients without previous gastric surgery served as control. Each patient underwent an oral glucose challenge. The patients with gastric resection underwent another glucose challenge with intravenous infusion to maintain the fluid volume. All patients with gastric resection showed subjective symptoms of the early dumping syndrome with significant (p < 0.001) increases (initial and maximum rates; mean +/- SD) in heart rate (from 70 +/- 3 to 122 +/- 4 beats/min) and in hematocrit (from 0.40 +/- 0.005 to 0.45 +/- 0.003). The plasma ANP level decreased significantly from 27.24 +/- 5.01 to 15.94 +/- 3.61 fmol/ml (p < 0.01). A significant negative correlation was found between the changes in hematocrit and the changes in plasma ANP level (r = 0.68; p < 0.001). Neither the subjective symptoms characteristic of the early dumping syndrome nor changes in laboratory parameters were noted in the patients during the challenge with infusion. The results show that the hypovolemia in dumping syndrome is associated with a significant decrease in ANP activity. The regulation of ANP release is also affected: apart from the well-known stimulating effect of hypervolemia, there exists an inhibition of secretion in volume-depleted states.