We showed immunohistochemically the localization of 5 alpha-reductase-containing cells in the rat brain, using a rabbit antibody generated against 5 alpha-reductase rat type 1. The antibody was produced by injecting the synthetic peptide corresponding to the amino acids 38-53 of 5 alpha-reductase rat type 1, conjugated to keyhole limpet hemocyanin with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide hydrochloride. Western blot analysis revealed that this antiserum recognized the protein with a molecular weight of 29,000 Da. The immunoreactive cells were distributed throughout the brain and they were preferentially located in the white matter rather than in the grey matter. These cells were mostly small and round and had a few fine processes. The immunoreaction was confined to the cytoplasm and processes. These findings indicate that 5 alpha-reductase rat type 1-containing cells are widely distributed in the rat brain and are located preferentially in the white matter rather than in the grey matter.