The effect of sulfite and arsenite on stomatal opening and light modulation of enzymes was examined in isolated epidermal strips of Pisum sativum L. var Little Marvel leaves. Sulfite or arsenite at 10 micromolar rapidly inhibited the stomatal opening process in light. Light activation of phosphoenolpyruvate carboxylase and NADP-malate dehydrogenase was completely diminished when the epidermal strips were incubated for 2 hours in light with either sulfite or arsenite at 10 micromolar. The data obtained suggest that the inhibition of stomatal opening by sulfite or arsenite in light might result from the inhibition of light modulation of key enzymes in guard cells.