Using a sensitive and specific radioimmunoassay for endothelin, we examined immunoreactive endothelin in human kidney tissue obtained from three necropsy and three nephrectomy cases. Immunoreactive endothelin was present in high concentrations in human kidney inner medulla (necropsy cases: 1 · 08 ± 0 · 47 pg/mg wet weight:mean ± SE)(nephrectomy cases: 2 · 77 ± 0 · 46). Characterization by reverse phase high performance liquid chromatography indicated that the only immunoreactive endothelin in human kidney inner medulla is endothelin-1, although immunoreactive endothelin in rat and pig kidney inner medulla comprises both isopeptides endothelin-1 and −3, suggesting that the genetic expression of endothelin differs according to species.