Abstract: Objective To study the therapeutic effect of baicalin(Bai)on Pneum ocystis carinii pneum onia(PCP)in rats.Methods PCP ratm odel was established by subcutaneous injection with dexamethasone sodium phosphate at a dose of 3.5 mg perrat,twice a week for 6 weeks.The PCP rats were orally admin istered with Baiat low dose(100m g/kg),mode rate dose(200 mg/kg)and high dose(400 mg/kg)once a day for 2 weeks.The therapy control group was orally adm inistered with sulfam ethoxazol(SMZ)250 mg/kg+trime thoprmi(TMP)50mg/kg.PCP model control group and normal control group were established,respectively.The therapeutic effect of Bai was evaluated by lung weight,lung/body weight ratio,cyst count and pathological changes.Results The lung weight of low dose group(2.26±0.14 g),moderate dose group(1.79±0.11 g)and high dose group(1.57±0.14 g)were lower than those of PCP model control group(2.70±0.15 g,P<0.01);the lung weight/body weight ratio of low dose group(1.32±0.22%),moderate dose group (1.04±0.13%)and high dose group(0.59±0.11%)were lower than those of PCP model control group (1.98±0.18%,P<0.01);the number of cysts of low dose group(42.86±5.01),moderate dose group(41.86±6.64) and high dose group(30.29±4.72)were lower than those of PCP model control group(56.29±5.82,P<0.01);the inflammation in lung tissues was remarkably lightened.The effect of high dose Bai group was smiilar to that of SMZco.Conclusion Bai had an obvious therapeutic effect smiilar to that of SMZco on Pneumocystis carinii pneumonia in rats.