Pisum sativum L. cv Alsweet (garden pea) and Lycopersicon esculentum flacca Mill. (tomato) were used to evaluate the phytotoxicity of SO2 and O3 in the light and dark. Plants were grown in controlled environment chambers and exposed to SO2 or O3 in the light or dark at the same environmental conditions at which they were grown. The pea plants were treated with fusicoccin to ensure open stomata in the dark; the stomata of the tomato mutant remained open in the dark. Both species exhibited 64% to 80% less foliar necrosis following exposure to SO2 (0.5 to 1.0 microliter per liter for 2 hours) in the light than in the dark. The decrease in SO2 injury for light versus dark exposed plants was greater in fully expanded than expanding leaves. Both species exhibited 30% greater foliar necrosis following exposure to O3 (0.2 microliter per liter for 2 hours) in the light than dark. The increase in O3 injury in the light versus dark was similar for leaves at all stages of expansion.