Mesophyll cells isolated fromPhaseolus vulgarisandLycopersicon esculentumshow decreasing photosynthetic rates when suspended in media containing increasing concentrations of osmoticum. The photosynthetic activity was sensitive to small changes in osmotic potential over a range of sorbitol concentrations from 0.44M(−1.08 MPa) to 0.77M(−1.88 MPa). Photorespiration assayed by14CO2release in CO2‐free air and by14CO2release from the oxidation of [1–14C] glycolate also decreased as the osmotic potential of the incubation medium was reduced. The CO2compensation points of the cells increased with increasing concentration of osmoticum from approximately 60 μ I−11 at −1.08 MPa to 130 μl 1−1for cells stressed at −1.88 MPa. Changes in photosynthetic and photorespiratory activities occurred at moderate osmotic potentials in these cells suggesting that in whole leaves during a reduction in water potential, non‐ stomatal inhibition of CO2assimilation and glycolate pathway metabolism occurs simultaneously wi