Neuromedin U is a novel neuropeptide found to have a widespread distribution extending throughout the mammalian central nervous system, gastrointestinal tract and the endocrine cells of the pituitary gland. In order to investigate the possibility that neuromedin U-like immunoreactivity is also present in the thyroid gland of the adult rat we have examined its localisation and molecular nature by radioimmunoassay, immunocytochemistry and chromatographic analysis. The neuromedin U content of the whole thyroid gland was found to be 331 +/- 67 fmol/gland (mean +/- SEM), and this value significantly decreased (163 +/- 17 fmol/gland) as a result of 14 days of treatment with the anti-thyroid agent methimazole (10 mg/rat/day. Thyrotoxicosis induced by exogenous T4 (10 micrograms/rat/day) failed to alter the thyroid content of this peptide. Immunostaining studies localised neuromedin U to a minor population of parafollicular C-cells in untreated animals. Complementary chromatographic studies revealed a single molecular form of neuromedin U-like immunoreactivity in thyroid tissue extracts which was indistinguishable from synthetic rat neuromedin U standard.