The orthophosphate inhibition of photosynthesis by isolated spinach chloroplasts can be reversed by 3-phosphoglycerate, dihydroxyacetone phosphate, glyceraldehyde 3-phosphate, fructose 6-phosphate and fructose 1,6-diphosphate. 2. Metabolically related compounds such as ribulose 1,5-diphosphate, glucose 6-phosphate, 6-phosphogluconate and phosphoenolpyruvate are ineffective. 3. The kinetics of reversal are characteristic of the intermediate used, but, in each instance, the onset of oxygen evolution is accompanied by a carbon dioxide fixation and except with 3-phosphoglycerate the stoicheiometry is close to unity. 4. The nature of orthophosphate inhibition and its reversal is discussed in relation to metabolic control of photosynthesis.
