In Mesembryanthemum crystallinum L. leaves exhibiting C_3 photosynthesis, crassulacean acid metabolism (CAM), as characterized by high PEP carboxylase and NADPmalic enzyme activities and malate synthesis in the dark, was induced by exposure to 400 mM NaCl, 400 mM KC1 or 400 mM Na_2S0_4 in the nutrient medium over two weeks. However, the activity of PEP carboxylase and NADP-malic enzyme in the 400 mM Na_2SO_4-treated plants were higher than those in the 400 mM NaCl- or 400 mM KCl-treated plants, though the leaf water potential in the 400 mM Na_2SO_4-treated plants was almost identical to that in the 400 mM NaCl- or 400 mM KCl-treated plants. The activities of PEP carboxylase and NADP-malic enzyme increased by exposing the plant roots to 140g・1^<-1> polyethylene glycol 6,000 (PEG) or by lowering the culture solution to the tips of roots as well. However, the enzymatic activities in plants which were exposed to 105g・1^<-1> PEG plus 100 mM NaCl were higher than those in the 140g・1^<-1> PEG-treated plants, though the leaf water potential in the 105g・1^<-1> PEG plus 100 mM NaCl-treated plants was almost identical to that in the 140g・1^-‘-treated plants. These results show that M. crystallinum shifts its mode of carbon assimilation from the C3 pathway to CAM in response to water stress, but this effect of water stress was enhanced by salt if the leaf water potential is low.