This study was carried out to investigate the effect of Yeemosan extract on the contractile force of the isolated guinea pig trachea smooth muscle and elucidate its mechanism. The results were obtained as follows: 1. The contractile response of the trachea smooth muscle of isolated guinea pig to acetylcholine was significantly inhibited by Yeemosan. 2. The contractile response of the trachea smooth muscle of isolated guinea pig pretreated propranolol was significantly inhibited by Yeemosan. 3. Effects of Yeemosan extract on the contractile response of the isolated guineapig trachea smooth muscle pretreated methylene blue was not significant. 4. The contractile response of the trachea smooth muscle of isolated guinea pig to prostaglandin $F2{alpha)$ was significantly inhibited by Yeemosan. 5. Effects of prostaglandin $F2{alpha)$ on the contractile response of the isolated guinea pig trachea smooth muscle pretreated Yeemosan was not significant. According to the above results. it was suggested that the contractile response mechanism of the guinea pig trachea smooth muscle to Yeemosan was related to sympathetic nervous system receptor and other mechanism should have further study.