Seedlings of five tree species of different shade tolerance were raised in full daylight and in three degrees of shade. Leaf chlorophyll content, the rate of respiration in darkness at 30°C and photosynthesis rates at 30°C over a range of light intensities, were measured in each case. The initial slope of the curve, relating photosynthesis rate and light intensity, provides a measure of the capacity of the photochemical system of the leaves and was usually steeper in shade grown leaves than in sun leaves. Thus all species showed a degree of adaptation to shade conditions, but, surprisingly, leaves of the shade tolerant species had slightly less steep slopes than the intolerant species. This was offset by a considerably lower rate of leaf respiration and a lower light compensation point in the tolerant species, so that considered over a range of varying low light intensities, the net photosynthesis would probably be similar in both groups. Moreover, whilst leaves of the tolerant species tended to show reduced respiratory activity when grown in shade, this was less evident in the intolerant species. The differences in rates of respiration may he the most important determinants of success or failure in woodland shade, where the plant may spend many more hours below than above the light compensation point.