A few reports suggested that guanidine increases the quantity of acetylcholine released from the nerve endings and increases the sensitivity of the end plate to acetylcholine. In this experiment the authors attempted to investigate the influence of guanidine on the acetylcholine effect on the blood pressure of rabbits and excised intestine. The results obtained we re summarized as follows. 1. The hypotensive effect of acetylcholine on the rabbit is augmented by pretreatment with guanidine 5-10 mg/kg. however is inhibited by 20 mg/kg of guanidine. 2. The contractile effect of acetylcholine on the excised intestine of rabbit and rat is potentiated by guanidine.