The addition of chymostatin (100 mg m'^) to the extraction and desalting media for phosphoenolpyruvate carboxylase (EC 4.1.1.31) from Zea mays L. and Setaria verticillata (L.) Beauv. (C4-plants) affected profoundly the kinetics of the enzymic activity, the rate curve became considerably more sigmoid than that of the control and, therefore, the activity at low phosphoenolpyruvate levels was much reduced. Also, the sensitivity towards malate inhibition was substantially increased in both the day and night forms of the enzyme. The above effects were observed at pH 7.0, but not at pH 8.0. Glucose-6-phosphate (5 mM in extraction and desalting media) partly counteracted the effect of chymostatin on malate sensitivity. Hence chymostatin affected directly or indirectly the enzymic dimer/tetramer equilibrium in favour of the dimer.The addition of chymostatin (100 mg m'^) to the extraction and desalting media for phosphoenolpyruvate carboxylase (EC 4.1.1.31) from Zea mays L. and Setaria verticillata (L.) Beauv. (C4-plants) affected profoundly the kinetics of the enzymic activity, the rate curve became considerably more sigmoid than that of the control and, therefore, the activity at low phosphoenolpyruvate levels was much reduced. Also, the sensitivity towards malate inhibition was substantially increased in both the day and night forms of the enzyme. The above effects were observed at pH 7.0, but not at pH 8.0. Glucose-6-phosphate (5 mM in extraction and desalting media) partly counteracted the effect of chymostatin on malate sensitivity. Hence chymostatin affected directly or indirectly the enzymic dimer/tetramer equilibrium in favour of the dimer.