Sucrose unloading and sink activity were examined in tomato plants (Lycopersicon esculentum) overexpressing sucrose phosphate synthase (SPS; EC Like the leaves, the fruit of the transformed tomato plants had elevated (2.4-fold) SPS activity. SPS over-expression in tomato fruit did not significantly change acid invertase, and only slightly reduced ADPglc ppase activity, but enhanced sucrose synthase activity by 27%. More importantly, the amount of sucrose unloaded into the fruit was considerably increased. Using [3H]- (fructosyl)-sucrose in in vitro unloading experiments with harvested 20-d-old fruit, 70% more sucrose was unloaded into the transformed fruits compared to the untransformed controls. Furthermore, the turnover of the sucrose unloaded into the fruit of transformed plants was 60% higher than that observed in the untransformed controls. Taken together, these results demonstrate that SPS overexpression increases the sink strength of transformed tomato fruit.