Abstract: In urethane anaesthetized cats, i. v. doses of bretylium 1-2 mg/kg produced a prompt fall of blood pressure. When the dosage was raised to 4 mg/kg, a short-lived marked hypotension was followed by a secondary hypotension of long duration. In small doses it slightly increased the force of cardiac contraction, but diminished it with larger doses. Unlike other quaternary bases which possess ganglion blocking action, bretylium exhibited highly specific blocking action on the adrenergic nerves, as shown by a marked reduction of the amount of transmitter substances released on stimulation of this nerve. However, larger doses of bretylium also blocked ganglia. The pressor and depressor effects of adrenaline and the pressor effect of noradrenaline were all enhanced by bretylium. The secretory function of adreno-medullary glands was not depressed by hypotensive doses of bretylium. The acute i.v. LD₅₀ in mice was found to be 15.0 mg/kg. In mice killed by the drug, respiration ceased before the heart. Oral doses of 100-200 mg/kg/day given for 4 weeks did neither affect the growth, nor produce other signs of poisoning. Among the 5 new compounds tested, which are structurally related to bretylium, the m- and p-isomers differed from bretylium, the o-compound, in their effects on the blood pressure of urethanized cats; they produced a marked rise of blood pressure after an initial slight drop of short duration. The rise of blood pressure was accompanied by contraction of nictitating membrances and salivary secretion. The depressor and salivatory effects of these isomers could be prevented by atropine pretreatment, while their pressor effect could be abolished by bretylium itself.