1. The recently described reaction of ATP-induced luminescence is analyzed for its relation to other ATP-induced reactions such as ATP-driven transmembrane proton gradient formation and ATP-driven reverse electron flow. 2. In the absence of phenazine methosulfate ATP-induced luminescence is optimal while the main phase of ATP-driven reverse electron flow is eliminated. 3. DCMU which by itself causes a much smaller luminescence, inhibits the ATP-induced luminescence. 4. Nigericin plus valinomycin, but not each by itself, fully inhibit the ATP-induced luminescence. 5. The observations are interpreted as indicating that ATP stimulates luminescence by a 2-fold mechanism: (a) increasing the amount of the reducing primary electron acceptor of Photosystem II, Q, and (b) creating a transmembrane electrochemical potential which serves to decrease the activation energy required for the charge recombination reaction which leads to luminescence.