Immunodeficient animals--the nude mouse and the nude rat--allow studies of drug action and possible side effects without interference from the immune system. Comparative investigations in athymic and euthymic animals allowed us to elucidate the role of T-lymphocytes in the pathogenesis of streptozotocin-induced diabetes mellitus in mice, and the importance of NK-cells as effectors in guanethidine-induced sympathectomy in the rat. It is suggested that immunodeficient animals should be included in toxicological studies of xenobiotics.