The respiratory oxygen uptake by mesophyll protoplasts of pea (Pisum sativum cv Arkel) was stimulated up to threefold after 15 minutes of illumination at an intensity of 1250 microeinsteins per square meter per second in the presence of 5 millimolar bicarbonate at 30°C. The extent of light-enhanced dark respiration (LEDR) increased progressively with duration of preillumination. The LEDR exhibited two phases. The initial high rate of respiration decreased in about 10 minutes to a lower steady value similar to that before illumination. The promotion of LEDR by the presence of bicarbonate and inhibition by glyceraldehyde or 3-(3,4-dichlorophenyl)-1,1-dimethylurea suggested that LEDR was dependent on products of photosynthetic carbon assimilation/electron transport. Thus, the photosynthetic products exert a markedly quick influence on dark respiration in mesophyll protoplasts.