The induction of Crassulacean acid metabolism (CAM) characterized by day/night acid fluctuation was measured in leaves from well-watered plants of Mesembryanthemum crystallinum grown in pots of four different volumes (160, 740, 2 600, and 6 500 cm^). CAM induction was dependent on soil volume: ťhe larger the pot, the later the induction took plače. Induction started when shoot water content fell to below 3 000 % of dry mass. For plants grown in the smaller pots (160-2 600 cm^) induction was linearly correlated with decreasing leaf water content. In contiast, induction was neither correlated with age nor with developmental stage of the plants. For plants grown in the largest pots leaf water content was not decreased and CAM was not induced up to week 10. Thus CAM induction is controlled environmentally rather than developmentally.