Under continuous light the endogenous Crassulacean acid metabolism (CAM) rhythm of Kalanchoe daigremontiana Hamet et Perrier de Ia Bâthie disappears at high (>29.0"C) or low (<8.0"C) temperatures. We investigated the reinitiation of rhythmicity when temperature was reduced from above the upper and increased from below the lower threshold leve1 via measurements of (a) short-term changes in carbon-isotope discrimination to illustrate shifts between C, and C, carboxylation in vivo, and (b) the malate sensitivity of phosphoenolpyruvate carboxylase (PEPC) in vitro. When the net C0,-exchange rhythm disappears at both temperatures, the instantaneous discrimination indicates low PEPC activity. Leaf malate concentration and osmolarity attain high and low values at low and high temperatures, respectively. After small temperature increases or reductions from the iow and high temperatures, respectively, the rhythm is reinitiated, with phases shifted by 180" relative to each other. This can be related to the contrasting low and high leaf malate concentrations due t o direct inhibition of PEPC and possibly also of the phosphorylation of PEPC by malate. The experimental results were satisfactorily simulated by a mathematical CAM-cycle model, with temperature acting only on the passive efflux of malate from the vacuole. We stress the important role of the tonoplast i n malate compartmentation and of malate itself for the reinitiation and generation of endogenous CAM rhythmicity.CAM is an ecophysiological adaptation of plants to limited availability of water or CO, (Kluge and Ting, 1978). At night CO, is fixed via the C, carboxylation enzyme PEPC, with the product, malate, stored in the vacuole. During the day the decarboxylation of malate releases CO,, which is refixed by the C, carboxylation enzyme Rubisco when the stomata are closed.