The effects of light, temperature, and anaesthetics on ATP levels in the leaves of Chenopodium rubrum and of Phaseolus vulgaris have been determined by an UV-enzymic method. ATP levels were found to vary inversely with temperature. Moreover, the anaesthetics, ether and ohloroform, were found to elevate leaf ATP. The effect of light was to reduce ATP levels although a tendency for increased ATP at high light intensities was detected. A correlation is deduced between elevated ATP level and decreased cytoplasmic viscosity, for cold, light, and anaesthetics. On the basis that cold, and some anaesthetic substances, have been shown to release plants from dormancy, it is suggested that ATP levels may play a fundamental role in vernalization and in the breaking of dormancy, and that the interaction of light may also be on this basis. An explanation is suggested for these effects, and their wider implications are discussed.