Garden peas (Pisum sativum L. cv Alsweet) and a tomato mutant (Lycopersicon esculentum Mill. var flacca) were sprayed with fusicoccin, a fungal toxin affecting membrane transport properties, before exposure to SO2 or O3. Tomatoes treated with 10 micromolar fusicoccin and exposed to SO2 (0.7 to 1.0 microliter per liter for 2 hours) exhibited 2 to 6 times more injury than untreated plants exposed to SO2. Peas treated with fusicoccin and exposed to O3 had less injury than untreated plants exposed to O3 (0.1 to 0.3 microliter per liter for 2 hours). Several lines of evidence suggested that the fusicoccin inhancement of SO2 injury is not the result of increased gas exchange, i.e. the tomato mutant has permanently open stomata under all conditions, and in peas fusicoccin had no effect on SO2 or H2O flux in plants exposed to 0.12 microliter per liter SO2. However, a 21% greater leaf conductance in fusicoccin treated versus untreated plants indicated the possibility of some differences in gas exchange for peas exposed to 1.0 microliter per liter SO2. (Copyright (c) Plant Physiol 1984.)