Preillumination of leaves of spinach, soybean and maize in the absence of CO2 greatly enhanced the capacity for fixing CO2 in an immediately following dark period. Lightenhanced dark CO2-fixation was further observed in isolated chloroplasts of spinach and soybean. When isolated chloroplasts were illuminated, CO2-fixing capacity in the subsequent dark period increased rapidly at first and later more slowly attaining a stationary value in about 20 min. When the light was turned off at this stage, the capacity decreased very rapidly becoming zero in about 10 min. The magnitude of the enhanced dark fixation and its decay in the dark were not influenced by the presence or absence of atmospheric oxygen. In both leaves and isolated chloroplasts, no significant change in oxygen (21%) occurred in distribution patterns of radioactivity in products fixed by photosynthetic, or light-enhanced, dark, 14CO2-fixation. In preilluminated leaves 14C was incorporated into sucrose in the subsequent dark period, indicating that the photosynthetic carbon reduction cycle is operating in light-enhanced dark fixation in higher plants.