Phosphoenolpyruvate carboxylase catalyses the primary assimilation of CO(2) in Crassulacean acid metabolism plants. It is activated by phosphorylation, and this plays a major role in setting the day-night pattern of metabolism in these plants. The key factor that controls the phosphorylation state of phosphoenolpyruvate carboxylase is the activity of phosphoenolpyruvate carboxylase kinase. Recent work on Crassulacean acid metabolism plants has established this enzyme as a novel protein kinase and has provided new insights into the regulation of protein phosphorylation. Phosphoenolpyruvate carboxylase kinase is controlled by synthesis and degradation in response to a circadian oscillator. The circadian control of phosphoenolpyruvate carboxylase kinase can be overridden by changes in metabolite levels. The primary effect of the circadian oscillator in this system may be at the level of the tonoplast, and changes in kinase expression may be secondary to circadian changes in the concentration of a metabolite, perhaps cytosolic malate.