Light activation of phosphoenolpyruvate carboxylase from the leaves of the C4 plant Setaria verticillata (L.) is more pronounced at low CO2 levels. The 2-fold activation observed at physiological ambient CO2 becomes 3.64-fold at 5 μL/L and completely abolished above 700 μL/L. When the stomata close under the influence of abscisic acid at 330 μL/L CO2, the extent of light activation is high (3.59-fold), probably because the increased diffusive resistance keeps the internal CO2 at much lower levels. Under darkness. CO2 and absicisic acid do not affect the extractable phosphoenolpyruvate carboxylase activity. Internal CO2 levels may determine phosphoenolpyruvate concentratio in the cytoplasm through the control of its utilization by phosphoenolpyruvate carboxylase. We have recently proposed (Samaras et al. 1988) that photosynthetically produced phosphoenolpyruvate could be an activator of the enzyme. It is therefore suggested that CO2 indirectly affects the activation state of phosphoenolpyruvate carboxylase by controlling the levels of phosphoenolpyruvate which may act as an activator.