Effects of Haenginbanhatang Extract on the Contraction of Isolated Guinea Pig Trachea Smooth Muscle. This study was carried out to investigate the effect of Haenginbanhatang 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 contratile response of the trachea smooth muscle of the guinea pig to histamine was significantly inhibited by Haenginbanhatang. 2. Effects of Haenginbanhatang extract on the contractile response of the isolated guinea pig smooth muscle pretreated propranolol was not significant. 3. The contractile response of isolated guinea pig smooth muscle pretreated methylene blue was significantly inhibited by Haenginbanhatang, 4. The contractile response of the trachea smooth muscle of the guinea pig to prostaglandin F2a was significantly inhibited by Haenginbanhatang. 5. Effect of prostaglandin F2a on the contractile response of guinea pig smooth muscle pretreated Haenginbanhatang was not significant. According to the above results, it was suggested that the contractile response mechanism of the guinea pig smooth muscle to Haenginbanhatang was related to cyclic GMP.