1. 1. [I- 14C]PGA is readily reduced by a sonically ruptured chloroplast system in light. This reduction is inhibited by threose 2,4-diphosphate. 2. 2. PGA accumulates when 14CO 2 is incubated in light with the sonically ruptured chloroplast system in the presence of threose 2,4-diphosphate. This indicates that PGA is in the major pathway of photosynthetic CO 2 fixation in the system. 3. 3. Threose diphosphate inhibition of the total 14CO 2 fixation by the carbon cycle of sonically ruptured spinach chloroplasts is not due to inhibition of carboxydismutase, but due primarily to inhibition of triose phosphate dehydrogenase which in turn limits the formation rate of the photosynthetic carbon cycle CO 2 acceptor, RuDP.