A fumigation experiment was conducted to study the tolerance mechanism of Taxus media cv.hicksii to formaldehyde(CH₂O) stress.A year old cutting seedlings of Taxus media cv.hicksii were grown in an airtight fumigation chamber of 1 m³,and five treatments with various CH₂O concentrations(0,5,10,20 and 40 mg·m^-3CH₂O)were arranged by a completely randomized experiment with three replications.The results showed:(1)No significant injury symptoms were observed in the appearance until 1-day 40 mg·m^-3 CH₂O fumigation was done,and the injury symptoms were strengthened with time.(2)With the increase in CH₂O concentration,the contents of malonic dialdehyde(MDA),free proline(Pro)and relative electrical conductivity were enhanced significantly,while the soluble sugars(SS) were increased initially and decreased after 5-day fumigation.(3)The activities of superoxide dismutase(SOD),catalase(CAT),polyphenoloxidase(PPO) and glutathione reductase(GR)were induced as the first police line of removing active oxygen at 5 mg·m^-3CH₂O,however,PPO responded most immediately.While under 10-20 mg·m^-3 CH₂O concentrations,six enzymes worked together to remove active oxygen.Under 40 mg·m^-3CH₂O stress,all tested enzymes were suppressed in different degrees.The activities of ascorbate peroxidase(APX),PPO and GR were significantly reduced as compared with CK,otherwise the activities of SOD,peroxidase(POD) and CAT were significantly increased in comparision with CK.The result indicated that under lower CH₂O concentrations(5-20 mg·m^-3),Taxus media cv.hicksii could stand the CH₂O stress mainly through enhancing the levels of osmoregulation substance and scavenging effect of oxygen free radical.Under 40 mg·m^-3CH₂O concentrations,the activities of APX,PPO and GR were significantly suppressed.Meanwhile cell membrane lipid peroxidation was strengthened and plants had gotten hurt.In summary,Taxus media cv.hicksii could adapt to 20 mg·m^-3CH₂O polluted environment with no obvious damages.The condusions offer foundation of theory for the follow-on experiments about CH₂O purification ability and metabolic machiney of Taxus media cv.hicksii.