An attempt has been made to determine why iodoacetamide at certain concentrations inhibits photosynthesis but does not affect respiration of the site of inhibition of both processes is considered to be the same enzyme, namely, D-glyceraldehyde-3-phosphate dehydrogenase. Evidence is presented indicating that the site of iodoacetamide inhibition in photosynthesis is solely the photosynthetic carbon cycle since the synthesis of reducing power (adenosine triphosphate and reduced triphosphopyridine nucleotide) and the evolution of O2 is not impaired. At a concentration of 5 x 10-5 [image], iodoacetamide is a strong inhibitor of CO2 fixation by the isolated spinach chloroplast and by the reconstituted system as defined by Arnon. At this concentration glyceraldehyde-3-phosphate dehydrogenase is not affected. Phospho-ribulokinase, an enzyme considered to be exclusively concerned with photosynthesis is sensitive in this range of inhibitor. It was concluded that at low concentrations of iodoacetamide, this phosphorylation is blocked thus stopping photosynthesis while at this same concentration all enzymes of respiration are unaffected. The significance of the lack of accumulation of ribulose-5-phosphate in poisoned intact cells is discussed.