Dry weights of whole fruit and of different fruit tissues, such as the mesocarp (with exocarp) and the endocarp (with seed), were accumulated on early (`Spring Lady'), midseason (`Flamecrest'), and late-maturing (`Cal Red') peach [Prunus persica (L.) Batsch] cultivars during the 1988 growing season. Seasonal relative growth rate (RGR) patterns of whole fruit showed two distinct phases for `Flamecrest' and `Cal Red'; however, `Spring Lady' did not exhibit two distinct RGR phases. The shift from phase I to phase II of the whole fruit RGR curve was related to an intersection of mesocarp and endocarp RGR curves, indicating a change of physiological sink activities in those fruit tissues in the later-maturing cultivars, but not in the early cultivar. Nonstructural carbohydrate compositional changes in concentration or content were similar in the three peach cultivars. Sucrose accounted for most of the seasonal increase in mesocarp nonstructural carbohydrate concentration. A sudden rise of sucrose was associated with the phase shift of the fruit RGR curves of the midseason and late-maturing cultivars, but not of the early maturing cultivar; however, in the early maturing cultivar, mesocarp compositional carbohydrate changes and, particularly, the sucrose increase, indicate that the physiological processes normally associated with the two phases exist in very early maturing fruit but are not associated distinctly with two separate RGR phases.