Abstract: In rabbits anaesthetized with urethane, intravenous injection of neostigmine neither prevented nor antagonized the hypotensive action of streptomycin. The hypotensive action still persisted after administration of the ganglion blocking agent, hexamethonium. The contraction of the rabbit heart in situ was depressed by streptomycin and bis-(α-hydroxy- streptomycyl)-amine. In the spinal rats and cats, streptomycin induced a fall in blood pressure. While streptomycin caused a fall of blood pressure in anaesthetized cat, the responses of the nictitating membrane to cervical superior sympathetic nerve preganglionic fibre were diminished. It is suggested that the mechanism of the hypotensive effect of streptomycin was chiefly due to the inhibitory effect on the heart and ganglion blocking action. Furthermore, the hypotensive mechanism of streptomycin was found to be partly related to bis-(α-hydroxystreptomycyl)-amine, a toxic derivative of streptomycin.