AbstractSince 1968, radioimmunoassays have been developed for major gastrointestinal hormones (gastrin, cholecystokinin, secretin, and gastric inhibitory peptide) and for many substances in the gastrointestinal tract that await full hormonal status (vasoactive intestinal peptide, motilin, pancreatic polypeptide, enteroglucagon, serotonin, prostaglandins, inter alia). Although reports between laboratories on the circulating levels of gastrin and gastric inhibitory polypeptide are in relatively good agreement, wide discrepancies exist in reported values of secretin and cholecystokinin. Reports on qualitative responses of these hormones to various stimuli, however, appear to be consistent between laboratories. A review of some of the studies of radioimmunoassay techniques for gastrointestinal hormones permits analysis of some of the technical problems that can be encountered.