The inhibitory effect of desaspidin is shown to depend on the oxidation-reduction state of the system investigated. The more reducing the conditions, the more desaspidin-inhibited the system and vice versa.The effect of desaspidin on noncyclic photophosphorylation induced by TPN(+), ferricyanide or DPIP is shown to be due to an uncoupling action.The decrease in effectiveness of desaspidin under oxidizing condition is shown to depend upon a photooxidation catalyzed by photosystem I of chloroplasts, which is prevented, but not reversed, by the presence of reducing agents, such as ascorbate + DPIP.These results are interpreted as indicating that, contrary to previous suggestions, desaspidin is a nonspecific uncoupler of all types of photophosphorylation.