1. 1. Proton uptake was induced by ATP in the dark following light triggering of ATPase activity in chloroplasts. The accumulated protons were released when ATPase activity was inhibited by the energy transfer inhibitor DIO-9. 2. 2. Approximately two protons were taken up for each ATP hydrolyzed at pH 8. A drop in H+/ATP ratio was caused by uncouplers such as NH4Cl and carbonyl cyanide p-trifluoromethoxyphenylhydrazone. These uncouplers caused an increase in the rate of ATP hydrolysis without a corresponding increase in proton uptake. 3. 3. The energy transfer inhibitor dicyclocarbodiimide inhibited both ATPase activity and the rate of proton uptake without changing the H+/ATP ratio. 4. 4. The antibiotic valinomycin caused an increase in the rate of both proton uptake and ATP hydrolysis without altering the ratio of H+/ATP. The H+/ATP ratio varied with changes in the external pH. The results were discussed in view of the chemiosmotic theory of oxidative and photosynthetic phosphorylation.