Tissue distributions of 4 antiadrenergic agents labeled with 125I were determined in dogs. [125I]o-iodobenzyldimethylethyl ammonium show highly selective uptake in the adrenal medulla. Studies of molecular structure-distribution indicate that both the nature of the cationic head and the ring position of the I atom greatly influence adrenal specificity. Distinct images of dogs'' adrenal medulla were obtained 4 days after i.v. injection of 1.5 mCi of [131I]o-iodobenzyldimethyl-2-hydroxyethyl ammonium.