(a) Spinach leaf discs were floated overnight on various concentrations of tentoxin, an ATP synthase inhibitor, or nigericin, an uncoupler. They were then illuminated in saturating CO 2. (b) With tentoxin, the inhibition of photosynthesis was accompanied by lower ATP/ADP ratios and increased ‘energy’ quenching of chlorophyll fluorescence. There was a small increase in the reduction of the Photosystem II acceptor, Q A, as monitored by photochemical quenching of chlorophyll fluorescence. However, activation of NADP-malate dehydrogenase decreased, showing that the acceptor side of Photosystem I becomes more oxidised, (c) With nigericin, the inhibition of photosynthesis was accompanied by decreased ATP/ADP ratios, decreased energy quenching, Q A became more reduced than with tentoxin, and the acceptor side of Photosystem I also became more reduced, (d) The results are used to discuss the contribution of “energy” quenching to energy dissipation, and the operation of photosynthetic control in leaves.