Phosphoenolpyruvate carboxylase (EC 4.1.1.31), partially purified from spinach (Spinacia oleracea) leaves, is inhibited by SO(3) (2-) ion. The inhibition is competitive or mixed type with respect to HCO(3) (-) (Ki = 17 mm), and noncompetitive with respect to phosphoenolpyruvic acid (Ki = 11 mm), Mg(2+) (Ki = 10 mm), and Mn(2+) (Ki = 2.4 mm). The inhibitory effect of SO(3) (2-) is more significant in the presence of Mn(2+) than in the presence of Mg(2+). l-Malate, an inhibitor of phosphoenolpyruvate carboxylase activity, and SO(3) (2-) may bind at the same site on the enzyme. Glyoxal bisulfite and glyoxylate bisulfite are equally effective inhibitors of the enzyme activity as SO(3) (2-), but alpha-hydroxypyridinemethanesulfonate is a weak inhibitor. The data are discussed in relation to the physiological effect of the air pollutant (SO(2)) on plant leaf metabolism.