The factors affecting stomatal conductance (gs) of I-214 (Populus euramericana) and a hybrid poplar, Peace (P. koreana x P. trichocarpa), were examined in the field and under controlled environment conditions. Unusual opening of the stomata was observed with Peace leaves at all positions. Ontogenetic changes in gs were similar between these two poplar species in the light. However, the dark/light ratio of gs in Peace poplar varied from 0.58 to 1.23 with the insertion level while that of I-214 poplar was zero except for the third leaf from the top. The gs of I-214 poplar changed with time of the day, varying from 0.74 mol m-2s-1 in the morning to zero at night, while the gs of Peace poplar changed only from the minimum value of 0.23 mol m-2s-1 at night to the maximum of 0.48 mol m-2s-1 in the morning. Under severe water stress, below -1.5 MPa, which decreased the gs of I-214 poplar to almost zero, the gs of Peace poplar remained about onethird of that observed with well-watered leaves. Exposure to a relatively high concentration of O3 caused the gs of I-214 poplar to decrease nearly to zero but had no effect on the gs of Peace. Stomata of Peace poplar were not affected by ABA and the gs did not change even with 10-4M ABA, while the gs of I-214 decreased to almost zero on the application of this concentration of ABA.