1. The pathophysiology of the dumping syndrome is poorly understood. Plasma levels of four small intestinal hormones have been measured after an oral glucose provocation test in 19 patients with dumping symptoms and in matched controls. 2. Plasma levels of neurotensin, a newly discovered highly potent, hypotensive ileal peptide, were significantly increased in symptomatic patients compared with those of controls [20 min rise of 43 ± 6.0 (mean ± sem) pmol/l in 19 symptomatic patients, 8.0 ± 5.5 pmol/l in 20 postoperative symptom-free patients, and 4.1 ± 3.5 pmol/l in 20 pre-operative patients with duodenal ulcer, P < 0.01]. 3. The rise in enteroglucagon was greater than normal but of similar magnitude to that seen in several other gastrointestinal conditions not associated with dumping symptoms. 4. The release of both gastric inhibitory peptide and motilin did not differ significantly from that of controls.