Phosphoenolpyruvate carboxylase (PEPC) was extracted from Mesembryanthemum crystallinum L. performing Crassulacean acid metabolism, at frequent intervals during a 12-hour light/12-hour dark cycle. Inhibition of PEPC by malate was followed at pH 8.0 and 7.5, 1 minute after homogenization of leaves. PEPC was more sensitive to malate during the light than during the dark periods and inhibition by malate was more pronounced at pH 7.5 than 8.0. For example, PEPC was not or only slightly inhibited by 0.5 millimolar malate during the dark period at both pH values and the rates per milligram chlorophyll were about the same. During the light period, 0.5 millimolar malate resulted in a 20 to 30% reduction of PEPC activity at pH 8.0 and a 80 to 90% reduction at pH 7.5. These and other experiments, in which plants were kept in prolonged dark periods, indicate that the increase in sensitivity of PEPC to malate is correlated with the change from acidification to deacidification in the tissue. These interactions account for apparent changes in pH response of PEPC in crude extracts assayed at different times of the day/night cycle.The experiments suggest the existence of two distinct states of PEPC in vivo, which differ in their susceptibility to malate inhibition during a day/night cycle. The change in properties is discussed in relation to the regulation of PEPC during Crassulacean acid metabolism.