Acer saccharum, Cornus amomum, Fraxinus americana, Juglans nigra, Acer rubrum, and Ulmus americana seedlings were subjected to a soil drying cycle. Twenty-four seedlings of each species were water stressed. At frequent intervals during the drying cycle, measurements were made of equilibrium transpiration rates, changes in transpiration resistance following changes in light intensity, and leaf water potential. Six plants of each species were used for gas exchange measurements, the remaining 18 for water potential determinations. Equilibrium transpiration rates of all plants decreased with increasing leaf water stress. Stomatal closure of Ulmus, Fraxinus and Juglans was particularly apparent at low water potentials, while the two Acer species maintained stomatal opening as the plants were water stressed. Stomata of the two Acer species and Ulmus were sensitive to small decreases in leaf water potential. Stomatal opening times (times to equilibrium transpiration resistance) following increases in light intensity were protracted at low leaf water potentials. In response to a decrease in light intensity, stomatal closing times of all species except Acer rubrum became significantly shorter as the plants were stressed. Following changes in light intensity, stomatal opening of Ulmus and Acer saccharum and stomatal closing of the two Acer species were most sensitive to increasing water stress. Stomatal opening of Ulmus and closing of Fraxinus were comparatively insensitive to low leaf water potentials. Transpiration resistance changes following changes in light intensity in stressed and unstressed plants are considered in relation to plant ecology. Forest Sci. 21:129-133.