The effects on photosynthetic metabolism in Chlorella pyrenoidosa and in isolated spinach chloroplasts of adding vitamin K 5 have been investigated. The most pronounced effect is the induction of the oxidative pentose phosphate cycle in the light, as indicated by the immediate appearance of 6-phosphogluconic acid upon the addition of vitamin K 5. The appearance of 6-phosphogluconic acid in isolated spinach chloroplasts demonstrates the operation of the oxidative pentose phosphate cycle in chloroplasts. The induction of the oxidative cycle is accompanied by inactivation of two enzymes of the photosynthetic carbon reduction cycle, or reductive pentose phosphate cycle. These enzymes are fructosediphosphatase (EC 18.104.22.168) and phosphoribulokinase (EC 22.214.171.124). Some inactivation of ribulosediphosphate carboxylase (EC 126.96.36.199) is also indicated. The inactivation of these enzymes may be related to the normal light-dark regulation of metabolism in vivo. Although it has been reported that addition of vitamin K 5 stimulates cyclic photophosphorylation in broken chloroplasts, such addition in the present experiments caused no increase in the level of ATP in photosynthesizing Chlorella or photosynthesizing intact spinach chloroplasts. A study of the changes of levels of labeled amino acids and carboxylic acids in photosynthesizing C. pyrenoidosa upon the addition of the inhibitor shows an increase in the levels of glutamic and citric acids and a decrease in the levels of aspartic and malic acids. These changes are interpreted as indicating a stimulated increase in the rate of oxidation of pyruvic acid to CO 2 and acetyl-CoA, which may increase the rate of conversion of oxaloacetic acid to citric acid, leading ultimately to the production of glutamic acid.