The leaf conductance responses of clones of Populus trichocarpa, P. deltoides, and hybrids to changing leaf water potential were examined. Unlike the other species, P. trichocarpa not acclimated to water stress showed an inability to control water loss at low leaf water potential. It is suggested that stomata of P. trichocarpa plants grown under well-watered conditions remain open in spite of a loss of turgor in the leaf. Preconditioning through water stress of developing leaves, however, leads to greater stomatal control in foliage following rewatering. Leaf conductance of stressed and rewatered P. deltoides and hybrids remained near zero, while the leaf conductance of P. trichocarpa was partially reduced.