The predictive capacity of two simulation models with different degrees of complexity for the calculation of assimilate production, was tested at different time scales, using a data set of wheat grown in an open-top-chamber experiment at two CO2 concentrations. Observed values of net canopy assimilation (Pn) were obtained from wheat plants grown at ambient (410 ?mol mol1) and elevated (680 ?mol mol1) CO2 mole fractions. Pn was simulated by using either simple multiple regression equations (AFRCWHEAT2) or by highly detailed calculations of leaf energy balances and the coupling of photosynthesis with stomatal conductance (LINTULCC2). Irrespective of the developmental stage of the crop or variation in weather, the models accurately simulated canopy assimilation and growth. We conclude that the response of aboveground-biomass production to elevated CO2 concentrations was explained primarily by the effects of CO2 on radiation-use efficiency and assimilate production. The models explained satisfactorily the daily course of Pn, its integrated daily totals, and the seasonally produced aboveground biomass, both at ambient and elevated CO2 concentrations. Specific problems in the simulations were identified and discussed.