We monitored sap flow and estimated diurnal changes in transpiration rates of two trembling aspen (Populus tremuloides Michx.) stands, located in the southern boreal forest and aspen parkland of Saskatchewan, Canada. In both stands during the peak growing season (June and July), sap flow during the day (0700–1700 h local time) increased linearly with vapor pressure deficit (VPD) from 0 to about 1 kPa, but then remained remarkably constant at VPD > 1 kPa (up to 4.8 kPa in the aspen parkland stand). The results imply an inverse relationship between stomatal conductance and VPD under well mixed conditions, for VPD > 1 kPa. We postulate that the stomata are operating to maintain leaf water potentials above a critical minimum value, which in turn places a maximum value on the rate of sap flow that can be sustained.