. In anesthetized rats, manganese (1 mg MnCl2 perkg·min) inhibited the calorigenic actions of adrenaline, noradrenaline or isoprenaline and the depressor responses to isoprenaline completely; calorigenic action of theophylline and positive chronotropic actions of all agonists were inhibited partially, and pressor responses to adrenaline and noradrenaline not at all. Smaller doses of manganese increased the pressor responses to adrenaline. Manganese inhibition was noncompetitive andcould not be overcome by artificial ventilation or calcium infusion,. respectively