Potential increases in plant productivity in response to increasing atmospheric CO(2) concentration are likely to be constrained by nutrient limitations. However, the interactive effects of nitrogen nutrition and CO(2) concentration on growth are difficult to define because both factors affect several aspects of growth, including photosynthesis, respiration, and leaf area. By expressing growth as a product of light intercepted and light use efficiency (epsilon), it is possible to decouple the effects of nutrient availability and CO(2) concentration on photosynthetic rates from their effects on other aspects of plant growth. I used measured responses of leaf photosynthesis to leaf nitrogen (N) content and CO(2) concentration to parameterize a model of canopy radiation absorption and photosynthesis, and then used the model to estimate the response of epsilon to elevated CO(2) concentration for Pinus radiata D. Don, Nothofagus fusca (Hook. f.) Ørst. and Eucalyptus grandis W. Hill ex Maiden. Down-regulation of photosynthesis at elevated CO(2) was represented as a reduction in either leaf N content or leaf Rubisco activity. The response of epsilon to elevated CO(2), which differed among the three species, was analyzed in terms of the underlying relationships between leaf photosynthesis and leaf N content. The response was independent of leaf N content when photosynthesis was down-regulated to the same extent at low and high leaf N content. Interactive effects of N availability and CO(2) on growth are thus likely to be the result of either differences in down-regulation of photosynthesis at low and high N availability or interactive effects of CO(2) and N availability on other aspects of plant growth.