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A big-leaf model of C3-canopy mass and energy exchange was used to predict hourly CO2 and O3 uptake by a mixed deciduous Quercus-Acer (oak-maple) stand in central Massachusetts, USA. The model is based on canopy-radiation interactions, leaf mesophyll metabolism (photosynthetic electron transport, carboxylation and oxygenation of ribulose 1,5-bisphosphate [RuP2] by RuP2 carboxylase/oxygenase [Rubisco], and respiration), physical transport conductances of mass and heat above and within the canopy, conductances of mass at the leaf surface and in the mesophyll, and mass and energy exchange at the soil surface (forest floor). Predictions of hourly CO2 and O3 uptake were compared to independent whole-forest CO2 and O3 exchange measurements made by the eddy correlation method during a 68 day period in the summer and early autumn of 1992. Predicted hourly CO2 exchange rate was strongly correlated (r ≈ +0.91) with measured hourly CO2 exchange, but mean day-time predicted whole-forest CO2 uptake was c. 13% (c. 1.13 μmol CO2 m-2 s-1) greater than CO2 uptake measured by eddy correlation. The model tended to overpredict CO2 uptake during late afternoon, but was accurate during the rest of the day. Predicted and measured O3 uptake rates also were positively correlated (r ≈ +0.76). The diurnal patterns of predicted and measured O3 uptake indicated that stomata1 conductance (gs) was accurately predicted during the morning, but in the afternoon the model overpredicted gs. This pattern was consistent with the overprediction of afternoon CO2 uptake, and suggested that a feedback inhibition of photosynthesis occurred in the afternoon. This might have been related to source-sink imbalance following several hours of photosynthesis. On the whole, and in spite of the simplifications inherent in the big-leaf representation of the canopy, the model is useful for predicting forest-environment interactions and for interpreting mass and energy exchange measurements.