Seedlings of five tree species of different shade tolerance were raissed in 100, 44, 17 and 3% of natural daylight. Growth analysis was carried out on data obtained from two harvests during the growing season. The net assimilation rates (E) of the more tolerant species were little higher in 100% daylight than in slight shade (44% daylight), but thereafter E decreased as shade increased. The intolerant species had greatest E in 100% daylight, but this high E fell sharply with increasing shade, and in Populus tremuloides was negative in 3% daylight. Even in this deep shade, however, the intolerant Liriodendron tulipifera had as high a net assimilation rate as the shade-tolerant species. Decreased E was offset by increased leaf area ratio (leaf area/plant weight), but in contrast to previous studies with herbaceous species, the magnitude of this increase gave no indication of the relative shade tolerance of the different species. In L. tulipifera and Acer rubrum, the increased leaf area ratio resulted from an increase in the proportion of assimilates diverted into leaf production (i.e. increased leaf weight ratio), together with a relatively greater increase in specific leaf area (leaf area/leaf weight). The more tolerant Fagus grandifolia and Quercus rubra showed less increase in specific leaf area, and leaf weight ratio increased only in the deepest shade. No clear-cut distinction can be drawn between tolerant and intolerant species on the basis of the relative size or efficiency of their photosynthetic systems in low light; factors other than those relating to carbohydrate economy in shade, also play a role.