Determinations of COa assimilation per unit leaf area in relation to light intensity were made with detached leaves of various physiological ages of the following species: Acer pseudoplatanus L., A. platanoides L., A. saccharinum L., Quercus robur L., and Q. borealis maxima (Marsh) Ashe. Measurements were made using the diaferometer technique, at a temperature of 25° C. and on first year material only. In general, maximum assimilation level and saturation light intensity increased with physiological age of plant material up to full leaf expansion and then fell off during senescence. In Quercus borealis, however, the maximum assimilation level was higher in young than in half-expanded leaves; it was concluded that, in this species, leaf expansion outstrips the rate of formation of an internal factor limiting photosynthesis, thus giving rise to an initial decrease in photosynthesis when expressed per unit leaf area. An experiment using seedlings of Acer saccharinum grown under different daylength conditions and, consequently, in different stages of development, demonstrated that physiological age is more important than actual age in determining the rate of photosynthesis. In general, the maximum recorded rates of assimilation were markedly higher in species of Acer than in the Quercus species. From a comparison of these results with those of previous workers it was concluded that variation in photosynthesis potential are at the specific rather than the generic level and are determined to a great extent by the cultural conditions, especially light intensity.