The activation and steady-state kinetics of a wheat sedoheptulose-bisphosphatase, shown to be located in the chloroplast, are examined. Enzyme activation in the presence of the reductant, dithiothreitol, is slow and modulated by the sedoheptulose 1,7-bisphosphate concentration. A mechanism for the regulation of enzyme activity is proposed. In this scheme, both the enzyme-substrate complex and the enzyme alone undergo a slow reductive activation and oxidative inactivation. The proportion of enzyme in the active form is governed by the reductant / oxidant ratio and the sedoheptulose 1,7-bisphosphate concentration. Control of sedoheptulose-bisphosphatase activity by these two factors is discussed in terms of regulation of the reductive pentose phosphate pathway of photosynthesis. Several observations made in previous studies using intact chloroplast systems are accounted for by the proposals outlined in this report.