Sorbitol and sucrose export out of mature peach (Prunus persica L. Batsch) leaves was studied by simulating carbon fluxes. Photosynthesis and 14C partitioning into sorbitol and sucrose were measured at the beginning of the photoperiod and the export rate of sorbitol and sucrose was modeled using saturable functions of the exportable carbohydrate content. The C fluxes were modeled using PSPICE (Simulation Program with Integrated Circuit Emphasis, University of Berkeley) simulator. The sorbitol and sucrose contents calculated by the simulator were compared with experimental carbohydrate contents. This allowed one to estimate the apparent Km and Vmax for sorbitol and sucrose phloem loading, and to quantify the effect of a theoretical small change in Km or Vmax on export rate. The estimated Vmax values for both sorbitol and sucrose phloem loading were similar to the photosynthetic C flux into each exportable carbohydrate measured under the leaf growth conditions. However, the fate of sorbitol differed from that of sucrose. The leaf appeared to have a higher storage capacity for sorbitol than for sucrose. The content of sucrose seemed tightly regulated.