Dawn stem water potentials of Quercus agrifolia, Juglans california, and Heteromeles arbutifolia were measured with a pressure chamber during the drought of 1972. Individuals of a particular species growing at the top of a slope had lower water potentials than those at the canyon bottom. These differences became greater as the season progressed and soil drying occurred. During the summer months when evaporative demands were high, leaf water deficits were not overcome during the overnight equilibration period as soil-plant water potential gradients existed in the dawn hours. Juglans and Heteromeles on the south-facing slope were able to maintain higher diurnal stem water potential maxima than corresponding individuals on the north-facing slope. This apparent anomaly may result from the more xeric conditions on the south-facing slope inducing acclimation or providing selection pressure for drought resistance of these two species.