Four cultivars of Phaseolus vulgaris were grown in a greenhouse and each flower was Labeled with date of anthesis. Seeds were collected at six different stages of development and inositol phosphates (InsPs) were analyzed by ion-pair reversed-phase HPLC. Phytate accumulation was similar in all cultivars, and the specific rate of phytate synthesis (Rs) peaked at about 22 days after flowering (DAF). Variations in the concentrations of the InsP3 and InsP4 pools matched changes in Rs in cultivars Una and Aruã. These results suggest mass-action effects. Thus, the rates of conversion of InsP3 to InsP5 appeared to be at least partly dependent on substrate concentration. Proportional increases in size of all InsP pools up to 21 DAF are also consistent with Little regulation in this part of the pathway. However, this did not appear to be the case in cv. Diamante Negro or with the conversion of InsP5 to InsP6 in all cultivars, where concentrations of the InsP precursor pools peaked earlier or even dropped as Rs peaked, suggesting activation of enzyme activity. Therefore, the evidence is consistent with a control point regulating this metabolic route upstream of InsP3 and possibly in the conversion of InsP5 to InsP6.