This study has been carried out to investigate the effects of Banhaonpaetang extract and its constituent herbs on the contractile force of Isolated Guinea pig trachea smooth muscle and to elucidate its mechanism. The results were as follows; 1. Banhaonpaetang significanly inhibitd the contractile force of isolated guinea pig trachea smooth muscle pretreated by histamine. 2. Banhaonpaetang significantly inhibited the contractile force of isolated guinea pig trachea smooth muscle pretreated by acetylcholine. 3. Radix Asari(細辛), Flos Inulae(旋覆花), Pericarpium Citri Nobilis(陳皮) and Radix Ginseng(人蔘) extract significantly inhibited the contractile force of isolated guinea pig trachea smooth muscle pretreated by histamine. 4. Radix Asari(細辛), Cortex Cinnamomi(桂心), Flos Inulae(旋覆花), Pericarpium Citri Nobilis(陳皮) and Radix Ginseng(人蔘) extract significantly inhibited the contractile force of isolated guinea pig trachea smooth muscle pretreated by acetylcholine. 5. The effects of Banhaonpaetang extract on the changes of plasma ACTH in rats were shown to be insignificant. 6. The effects of Banhaonpaetang extract on the changes of serum electrolytes were shown to be significant in $K^+$ and $Ca^{2+}$.