Ozone, leaf age and water stress each affected leaf conductance in soybean [Glycine max(L.) Merr. Hodgson], but there were no interactions among these factors. Exposure to increased concentrations of O3(0.01, 0.05, 0.09. and 0.13 μl l−1) resulted in linear declines in abaxial and adaxial conductances in leaves of all ages. There were no differences in relative response to O3between the two leaf surfaces. For well‐watered plants, water use efficiency also decreased with exposure to increased O3concentrations (water‐stressed plants were not tested). Abaxial conductance increased as leaves aged from 4 to 10 days and then declined with further aging. Adaxial conductance decreased with all increases in leaf age beyond 4 days, and the ratio of abaxial/adaxial conductance increased continuously throughout the leaf lifespan. During water‐stress cycles (water withheld for 2–3 days) leaves of water‐stressed plants had lower conductances than those from well‐watered plants, and there was no difference in relative response between abaxial and a