The serotonin level in patients after a 2/3 resection of the stomach (Billroth II) is normal to moderately raised. Patients with a dumping syndrome show no significantly different figures for the excretion of free serotonin in the 24-hr urine. The level of serotonin in patients after gastric operation is regulated to normal or slightly raised values by increased serotonin production in the gastrointestinal tract whether a dumping syndrome develops or not. Knowledge of the serotonin levels is consequently of therapeutic significance because a dumping syndrome may exist with and without raised serotonin values.