Abstract: The defense conditioned reflex was partially abolished and its latent period was pro- longed by intraperitoneal injection of 10mg/kg of sinomenine in mice, but the differen- tiation and unconditioned reflex still remained. In cats intraperitoneal injection of 5mg/kg of sinomenine significantly prolonged the defense conditioned reflex time, the defense conditioned reflex was partially abolished, and the unconditioned reflex remained normal or was partially abolished. When the dose was increased to 10mg/kg, the inhibitory effect was more pronounced. These results suggest that sinomenine may decrease the excitability of the cerebral cortex. In three cats, the acute toxicity of 5-10 mg/kg of sinomenine was shown as fol- lows: hyperactivity, salivation, the scratching with claws the periphery of their mouths and ears and intertoes, urination, excrement, tachypnea, and inhibition successively within 1/2 hour; these signs disappeared at 1-3 hours later. When the dose was raised to 75- 100mg/kg, the animals produced ataxia, convulsion, stupor, deep inhibition of respira- tion, and death within 105 to 180 minutes.