To evaluate effects of Pai Mo San (PMS) water extract on the plasma cortisol concentration and arterial blood $PCO_2,;PO_2$ in the rabbit. The results obtained were as follows: 1. Intravenous administration of PMS water extract at the dose of 0.2 ml/kg remarkably increased plasma cortisol concentration from 1 to 2 hours. 2. Intravenous administration of PMS water extract at the dose of 0.4 ml/kg plasma cortisol concentration showed more significant increase than the control group after 1,2,4 hours. 3. No change observed in the arterial blood $PO_2$ after intravenous administration of PMS water extract at the dose of 0.2 ml/kg used. 4. No change observed in the arterial blood $PO_2$ after intravenous administration of PMS water extract at the dose of 0.4 ml/kg used. 5. Intravenous administration of PMS water extract at the dose of 0.2 ml/kg arterial blood $PCO_2$ showed more significant decreased than the control group after 1,3 hours. 6. Intravenous administration of PMS water extract at the dose of 0.4 ml/kg significantly decreased arterial blood $PCO_2$. These results suggest that the therapeutic action of Pai Mo San water extract for Yeol Su (熱嗽), Wha Su (火熱), Gu Su (久熱) and anti-asthma maybe related with the increase of plasma cortisol, decrease of $PCO_2$.