The serotonin content of duodenum, as determined fluorimetrically after appropriate extractions, was found to be significantly decreased in rats after pyloroplasty and vagotomy and after gastrectomy with or without vagotomy. The serotonin content of the jejunum tended to parallel the duodenal changes but jejunal serotonin was not always significantly altered. Decreased serotonin content of the proximal small bowel is consistent with increased release of intestinal serotonin following operations which destroy or bypass the pyloric sphincter mechanism.