The behavior of 2,3,5,6-tetramethyl-p-phenylendiamine (DAD) in photosynthetic reactions of isolated chloroplast fragments was compared with that of N-tetramethyl-p-phenylendiamine (TMPD). Both reverse the DCMU-inhibition of photosynthetic NADP-reduction. The DAD-system (at high concentrations of DAD), is coupled to a stoichiometric ATP-formation, whereas the TMPD-system is not. This shows that p-phenylendiamines, depending on their constitution, may react with components of the electron transport chain of chloroplasts before or after the phosphorylation site, and locates the phosphorylation step of photosynthetic phosphorylation between two endogenous compounds in that part of the electron transport chain, which connects the two light reactions of photosynthesis. At lower concentrations of DAD the diminished NADP-reduction is no longer coupled to ATP-formation, indicating a second point of entry of electrons from DAD into the electron transport chain. DAD, furthermore, is a cofactor of cyclic photophosphorylation. It therefore behaves like DCPIP, but the rates of the DAD-system are higher.