Soybean plants were grown continuously at 350 and 700cm3m−3 CO2 at constant temperature. Respiration rates of third trifoliolate leaves were measured at the growth CO2 concentration for the whole dark period from 5d before through to 5d after full area expansion. The short-term response of respiration rate to the measurement CO2 concentration was also determined at each age. Respiration rates per unit of dry mass declined with age and were significantly less at a given age or RGR in leaves grown and measured at the elevated CO2. The difference in respiration rate was largest in mature leaves and resulted from the different measurement CO2 concentrations. The respiratory costs of the tissue synthesis, estimated from the elemental composition of the tissue, did not differ substantially between CO2 treatments. The response of respiration rate to carbon dioxide concentration was not strongly affected by the form of nitrogen supplied. Maintenance respiration calculated by subtracting growth respiration from total respiration was negative in rapidly growing leaves for both CO2 treatments. This indicates that CO2 efflux in the dark does not accurately reflect the average 24 h rate of energy expenditure on growth and maintenance for soybean leaves.