To evaluate daytime and nighttime carbon balance and assimilate export in soybean (Glycine max [L.] Merrill) leaves at different photon flux densities, rates of CO2 exchange, specific leaf weights, and concentrations of sucrose and starch were measured at intervals in leaves of pod-bearing `Amsoy 71' and `Wells II' plants grown in a controlled environment room. Assimilate export was estimated from CO2 exchange and change in specific leaf weight. Total diurnal assimilate export was similar for both cultivars. Large cultivar differences existed, however, in the partitioning of carbon into starch reserves and the relative amounts of assimilate exported during the day and the night. Total amounts of both daytime and nighttime export increased with increasing photon flux density, as did sucrose and starch concentrations, specific leaf weight, and rate of respiratory carbohydrate loss at night. Cultivar differences in nighttime rate of export were more closely related to the differences in amount of assimilate available at the end of the day than to differences in daytime rate of net CO2 assimilation. Daytime rates of export, however, were closely related to daytime rates of net CO2 assimilation within each cultivar. The total amount of starch depleted during the 10-hour night increased as starch concentration at the beginning of the night increased.