The construction of diel leaf carbon budgets, together with analyses of the δ¹³C composition of biochemical fractions, was used to examine how crassulacean acid metabolism (CAM) plants adjust carbohydrate partitioning in response to shifting sink demands. For Mesembryanthemum crystallinum L., net carbon budgets indicated clear shifts in assimilate partitioning and in the relative proportions of day : night export as CAM was induced. Different patterns of carbohydrate partitioning in primary and axillary leaves of this species may reflect the different sink priorities of determinate and indeterminate growth. In primary leaves, the high level of diel starch turnover may be a strategy for ensuring production of a critical mass of juvenile tissue that poises the plant for CAM induction. In axillary leaves, the high day-night flux through soluble sugars may ensure ready availability of assimilates for export to reproductive sinks. Carbon isotope ratios were measured for various organic fractions isolated from leaves and fruits of two species of Clusia that differ in CAM expression. Similar and C₃-like isotopic signatures were obtained for the structural material isolated from fruits of Clusia minor L. and Clusia rosea Jacq. The data suggest that the partitioning of C₄- and C₃-derived assimilates into discrete storage and transport pools of soluble sugars will ensure reproductive output, regardless of the level of CAM that is induced in these species.