From the maximum rate of malate accumulation in Vicia faba L. guard cells during stomatal opening the maximum rate of organic anion synthesis is calculated to be 200 millimoles per kilogram dry weight per hour. A minimum estimate for the phosphoenolpyruvate (PEP) carboxylase-catalyzed reaction in guard cells is 650 millimoles per kilogram dry weight per hour which is significantly higher than in any other leaf tissue. The apparent K(mpep) of the guard cell enzyme is 60 mum at pH 8.7, but is probably higher at lower pH. The concentration of PEP in guard cells was 270mum (=2.2 x 10(-15) moles/guard cell pair) during anion synthesis. These results support the possibility that the carboxylation of PEP is the anaplerotic step in guard cells.