In the present paper the development andapplication of a novel thrittene radioimmunoassay (RIA)are described. 125I-labeling of Tyr(0)-thrittene was performedby the iodogen-method and the mono-iodinatedpeptide, as RIA tracer, was separated by reversed-phasehigh performance liquid chromatography (HPLC). TheRIA results show that the antiserum used in the radioimmunoassayturned to be C-terminal specific, without significantaffinity to other members of the somatostatinpeptide hormone family. Detection limit of the assay was0.2 fmol/ml. This highly specific and sensitive thritteneRIA was used to investigate the distribution of thrittene inthe rat gastrointestinal tract and other tissue samples. Differentareas of the gastrointestinal tract and other tissueswere removed from rats and after extraction the sampleswere processed for thrittene radioimmunoassay. Highestconcentrations were found in the duodenum samplesfollowed by jejunum and ileum, however, all the examinedtissues contained highly enough thrittene for themeasurement