Day respiration is the process by which nonphotorespiratory CO2 is produced by illuminated leaves. The biological function of day respiratory metabolism is a major conundrum of plant photosynthesis research: because the rate of CO2 evolution is partly inhibited in the light, it is viewed as either detrimental to plant carbon balance or necessary for photosynthesis operation (e.g., in providing cytoplasmic ATP for sucrose synthesis). Systematic variations in the rate of day respiration under contrasting environmental conditions have been used to elucidate the metabolic rationale of respiration in the light. Using isotopic techniques, we show that both glycolysis and the tricarboxylic acid cycle activities are inversely related to the ambient CO2/O2 ratio: day respiratory metabolism is enhanced under high photorespiratory (low CO2) conditions. Such a relationship also correlates with the dihydroxyacetone phosphate/Glc-6-P ratio, suggesting that photosynthetic products exert a control on day respiration. Thus, day respiration is normally inhibited by phosphoryl (ATP/ADP) and reductive (NADH/NAD) poise but is up-regulated by photorespiration. Such an effect may be related to the need for NH2 transfers during the recovery of photorespiratory cycle intermediates.