The activity of the phosphoenolpyruvate (PEP) carboxylase isolated from non-saline grown plants of the salt-tolerant plant Mesembryanthemum crystallinum is strongly inhibited by malate. This inhibition was found to depend on the pH (rising with increasing H(+)-concentrations) and on the concentration of malate used. The addition of inorganic phosphate (appropriate concentration 30 mM) to the in-vitro enzyme assay prior to malate addition results in a remarkable compensation of malate-caused inhibition of the enzyme activity. Again a dependency upon the pH can be observed. The ability of inorganic phosphate to restore malate-caused inhibition of the PEP-carboxylase increases with increasing pH.Another potent inhibitor of the PEP-carboxylase is NaCl, which shows a minimum inhibition at pH 7. At this pH a concentration of more than 60 mM NaCl is needed to reduce the activity of the enzyme below the control level with a 50% inhibition is reached at 150 mM. If the addition of NaCl is performed in the presence of 30 mM inorganic phosphate the inhibition is less pronounced. The enzyme now tolerates about 100 mM higher concentrations of NaCl without being inhibited.NaCl-treatment of Mesembryanthemum crystallinum plants results in an increase of inorganic phosphate in the cells with a concomitant establishment of a Crassulacean acid metabolism. The present results support evidence for a protective function of inorganic phosphate (compensation of NaCl-induced enzyme inhibition), possibly a commen reaction involved in the question of salt tolerance, and a more specific function (restriction of malatecaused inhibition of the PEP-carboxylase) providing the enzymatic background for the malate accumulation in Mesembryanthemum crystallinum.