The integrated response of the regulatory peptides of the gastrointestinal tract to a test meal was studied in six hypercholesterolaemic patients who had undergone partial ileal bypass (PIB) and compared with responses in ten age- and sex-matched controls. Plasma motilin was found to rise significantly whereas plasma pancreatic polypeptide release was impaired after PIB. Plasma cholecystokinin rose promptly in both controls and the partial ileal bypass patients, and the latter group showed marked further rises, at 60 and 120 minutes after the meal, which were significantly higher than controls. Basal and postprandial plasma gastrin, gastric inhibitory peptide, neurotensin, enteroglucagon, insulin and pancreatic glucagon were similar in the two groups.