A controlled-environment experiment was initiated to evaluate the influence of CO2 enrichment on the growth and ontogeny of bigleaf maple (Acer macrophyllum). Development of seedlings was monitored from seed germination through the first five months of ontogeny in growth chambers containing 350 (ambient), 575 and 700 ppm CO2. Seedling shoot elongation, leaf and branch expansion, and foliage retention were altered by ambient CO2 conditions. These preliminary morphological responses are consistent with reports from previous experiments which employed tree seedlings pre-conditioned in ambient CO2 conditions. Alterations in seedling crown and leaf morphology due to CO2 enrichment may influence future adaptive responses of tree species to environmental stress agents.