Leaves of the Crassulacean acid metabolism plant Kalanchoe daigremontiana Hamet et Perr., about 3.3 mm thick, showed higher rates of net CO2 exchange through the lower than through the upper surface during day and night, although the lower surface received only a small fraction of the light which was incident on the upper surface. Nocturnal acidification was more pronounced in cells from the lower than from the upper portion of leaves. The lower activity of the exposed side of these long-lived succulent leaves may be related to the potentially adverse effects of excessive light.