SummaryA radioimmnunoassay for the measurement of immunoreactive secretin has been developed. The assay is specific and sensitive so that 2·8 fmol/ml can be measured. Basal secretin levels inn man are generally undetectable and a protein‐rich meal does not elicit a secretin reponse. However, the intraduodenal instillation of HCl leads to a prompt rise in circulating immunoreactive secretin and exogenous secretin administered intravenously is easily and accurately measured.