Quantum yields were measured for CO(2) fixation by sunflower (Helianthus annuus L.) leaves having various water potentials and for dichlorophenolindophenol photoreduction by chloroplasts isolated from similar leaves having various water potentials. In red radiation, the quantum yield for CO(2) was 0.076 for an attached sunflower leaf at a water potential of -3 to -4 bars but was 0.020 for the same leaf at -15.3 bars. After recovery to a water potential of -5 bars, the quantum yield rose to 0.060. Soybean (Glycine max L. [Merr.]) leaves behaved similarly. Chloroplasts from a sunflower leaf with a water potential of -3.6 bars had a quantum yield for 4 equivalents of 0.079, but when tissue from the same leaf had a water potential of -14.8 bars, the quantum yield of the chloroplasts decreased to 0.028. The decrease could not be attributed to differences in rates of respiration by the leaves or the chlorophyll content or absorption spectrum of the leaves and chloroplasts.The data are the first to demonstrate an effect of low leaf water potential on the quantum yield and they indicate that changes occurred close to the primary photochemical events of photosynthesis. The similarity in response of the leaves and chloroplasts indicates that certain changes in photosynthesis at low water potentials are attributable to the chloroplasts rather than the stomata.