Substituted p-phenylendiamines have only weak activity as cofactors of aerobic photophosphorylation in isolated chloroplasts. However, they reverse the inhibition by DCMU of photosynthetic NADP reduction and of quinone stimulated ascorbate photo-oxidation. This indicates, that phenylendiamines function as electrondonors at the level of cytochromes and in this respect resemble indophenols. In contrast to the indophenol system, however, photosynthetic NADP reduction and ascorbate photo-oxidation in a phenylendiamine system are not coupled to ATP formation. This seems to exclude a phosphorylation step between the first light reaction of photosynthesis and NADP reduction.