Abstract: Intravenous injection of Sinomenine hydrochloride to anaesthetized animals (dogs, rats, and rabbits) produced apparent maximal hypotensive effects. In dogs (1 mg/kg), the latent period of hypotension was 22±2.3 seconds; the hypotensive area was -40±3% and the action lasted 25-100 minutes. Repeated administrations revealed tachyphylaxis. The same results were obtained in rats, but the fall of arterial pressure was only transient in rabbits. Our experiments indicate that Sinomenine apparently inhibits the pressor responses induced by the occlusion of the common carotid arteries and the stimulation of the cen- tral ends of sciatic or vagus nerves. It inhibits the effects of stimulation of the distal end of vagus nerve, the effects of excitment of respiration, and the pressor effect by lobeline. Sinomenine also reduces the hypotensive actions of adrenaline and noradrena- line. The hypotensive action of Sinomenine is not due to histamine liberation. Sino- menine exerts no influence on the M-cholinergic reaction system or the cardiac depres- sion. Therefore it is suggested that the hypotensive effects of Sinomenine are mainly due to the central inhibition, the adrenolytic effects, and probably also, the ganglionic block- ing action.