Ephedradines A, B, C and D (3 mg/kg, i.v.) elicited hypotensive effects in Wistar rats. Administration of ephedradine B (EB) (0.1-3 mg/kg, i.v.) to Wistar rats and to spontaneously hypertensive rats reduced blood pressure in a dose-dependent manner. EB (1 mg/kg, i.v.) slightly potentiated the pressor action of norepinephrine (1 µg/kg, i.v.) and significantly reduced that of 1,1-dimethyl-4-phenylpiperazinium (50 µg/kg, i.v.). The hypotensive activity of EB (1 mg/kg, i.v.) was inhibited by pretreatment with atropine (5 mg/kg, i.v.) or diphenhydramine (5 mg/kg, i.v.). In the hypogastric nerve-vas deferens of guinea pig, application of EB (3 × 10-7-10-5 g/ml)to the ganglion inhibited the contraction of the vas deferens induced by electrical preganglionic nerve stimulation and by acetylcholine (10-4-10-3 g/ml) applied to the ganglion. It is thus concluded that the hypotensive activity of ephedradine B is exerted mainly by ganglion block.