Phosphoenolpyruvate (PEP) carboxylase is the primary CO2-fixing enzyme in plants which exhibit crassulacean acid metabolism. The activity of this enzyme is under the control of an endogenous circadian rhythm. A reversible phosphorylation event was proposed to control PEP carboxylase activity by altering its sensitivity to inhibition by the allosteric inhibitor, malate. The work described in this thesis aimed to investigate the regulation of PEP carboxylase and to gain further insight into the biochemical nature of this circadian clock.