Dynamics of fruit growth and photoassimilate translocation in tomato plants (Lycopersicon esculentum Mill.) as affected by irradiation, air temperature and water deficit were analyzed with reference to water relations, respiration and photosynthesis in fruits and leaves. A multiple chamber gas exchange system for intact fruits and leaves was newly developed for on-line measurements of transpiration, respiration and photosynthesis. Fruit growth rate and pedicel sap fluxes through phloem and xylem were evaluated on-line by applying a pair of laser sensors and the heat-ring method. Furthermore, photoassimilate flux through a pedicel was evaluated on the basis of sugar balance in ethylenediaminetetraacetic acid solution into which the cut end of the pedicel was immersed. Fruit expansive growth was attributed to 80% of the pedicel sap flux, while 20% of the sap flux was lost by transpiration from the fruit (pedicel, calyx and berry). Irradiation and temperature rise in the daytime activated leaf photosynthesis, fruit respiration and phloem and xylem sap fluxes, which resulted in remarkable increases in fruit growth and photoasssimilate translocation in the daytime. In particular, the activated fruit respiration contributed to fruit growth and photoassimilate accumulation through energy-dependent postphloem transport in the fruit. Water deficit brought depressions in fruit growth and photoassimilate translocation with decreases in phloem and xylem sap fluxes, fruit respiration and leaf photosynthesis, where sap backflow through xylem and fruit shrinkage were also found. Thus, the methodologies newly developed in this study can be useful for obtaining quantitative information on dynamics of fruit growth and photoassimilate translocation, which is essential for the optimization of greenhouse environment to improve yield and quality of tomato fruits.