Experiments were carried out to investigate whether sucrose synthase (Susy) catalyses a readily reversible reaction in vivo in potato (Solanum tuberosum L.) tubers, Ricinus communis L. cotyledons, and heterotrophic Chenopodium rubrum L. cell-suspension cultures. (i) The contents of sucrose, fructose, UDP and UDP-glucose were measured and the mass-action ratio compared with the theoretical equilibrium constant. In all three tissues the values were similar. (ii) Evidence for rapid turnover of label in the sucrose pool was obtained in pulse-chase experiments with potato discs and with intact tubers attached to the plant. The unidirectional rates of sucrose synthesis and degradation were considerably higher than the net flux through the sucrose pool in the tubers. (iii) Labelling of the glucosyl and fructosyl moieties of sucrose from [(14)C]glucose in the presence of unlabelled fructose provided evidence that Susy contributes to the movement of label into sucrose. Methods for estimating the contribution of sucrose-phosphate synthase and Susy are presented and it is shown that their relative contribution varies. For example, the contribution of Susy is high in developing tubers and is negligible in harvested tubers which contain low Susy activity. (iv) The absolute values of the forward (v(+1)) and backward (v(-1)) reaction direction of Susy are calculated from the kinetic labelling data. The estimated values of v(+1) and v(-1) are comparable, and much higher than the net flux through the sucrose pool. (v) The estimated concentrations of the substrates and products of Susy in tubers are comparable to the published K m values for potato-tuber Susy. (vi) It is concluded that Susy catalyses a readily reversible reaction in vivo and the relevance of this conclusion is discussed with respect to the regulation of sucrose breakdown and the role of Susy in phloem unloading.