We had previously demonstrated that expression of a cytoplasmic-localized ADPglucose pyrophosphorylase (AGPase) mutant gene from Escherichia coli in rice endosperm resulted in enhanced starch synthesis and, in turn, higher seed weights. In this study, the levels of the major primary carbon metabolites were assessed in wild type and four transgenic CS8 rice lines expressing 3- to 6-fold higher AGPase activity. Consistent with the increase in AGPase activity, all four transgenic CS8 lines showed elevated levels of ADPglucose (ADPglc) although the extent of increases in this metabolite was much higher than the extent of increases in starch as measured by seed weight. Surprisingly, the levels of several other key intermediates were significantly altered. Glucose 1-phosphate (Glc 1-P), a substrate of the AGPase reaction, as well as UDPglucose and Glc 6-P were also elevated to the same relative extent in the transgenic lines compared with the wild-type control. Analysis of metabolite ratios showed no significant differences between the wild type and transgenic lines, indicating that the reactions leading from sucrose metabolism to ADPglc formation were in near equilibrium. Moreover, glucose and fructose levels were also elevated in three transgenic lines that showed the largest differences in metabolites and seed weight over the wild type, suggesting the induction of invertase. Overall, the results indicate that the AGPase-catalyzed reaction is no longer limiting in the transgenic lines, and constraints on carbon flux into starch are downstream of ADPglc formation, resulting in an elevation of precursors upstream of ADPglc formation.