The aim of this work was to investigate whether product inhibiton of sucrose-phosphate synthase (SPS) by sucrose-6-phosphate (sucrose6P) plays a significant role in feedback regulation of sucrose synthesis in spinach leaves. The amount of sucrose6P in leaves was very low (0.6-0.7 nmol mg −1 chlorophyll) and did not increase during the day when sucrose accumulated in the leaf. Sucrose6P did not have any significant inhibitory effect on partially purified spinach leaf SPS, and investigation of the reverse reaction revealed a high apparent K m of 6 mM for sucrose6P. These results, together with previous measurements of cytosolic metabolite levels in spinach leaves, were used to calculate improved values for the in vivo free energy changes of the reactions catalysed by SPS and sucrose-6-phosphatase. Both reactions are significantly displaced from equilibrium. It is concluded that direct product inhibition by sucrose6P does not play an important role in the regulation of spinach leaf SPS.