Immunohistochemical localization of mineralocorticoid receptor in the human adult gut was examined by using a polyclonal antibody raised against a mineralocorticoid receptor fusion protein. Immunoreactive staining was observed in columnar cells in the middle of the intestinal glands of the proximal colon, but not in goblet cells. Western blot analysis also revealed the presence of an approximately 100 kDa immunoreactive species consistent with the mineralocorticoid receptor protein in the ascending colon as well as the kidney. These results suggest that aldosterone-sensitive sodium transport and water absorption may be mainly carried out in the proximal colon in human adult gut at non-pathological state.