To study the effect of removing tubers on growth and net assimilation rate (E) of potato, plants were grown in pots partly filled with soil with the shoot growing through a polythene cover. Tubers developed in the space between the cover and the soil surface. Removing tubers immediately they began to form had little effect on E at the beginning of the experiment but later greatly reduced it. Shading reduced E more at the beginning of the experiment than later. Removing tubers decreased total dry weight, but much of the material that would have moved to tubers accumulated in leaves and stems. In intact plants the loss of weight by shading was mainly from the tubers; in plants without tubers it was mainly from stems and leaves. Removing tubers increased leaves on lateral stems. Increasing the amount of nitrogen supplied diminished the effect on E of removing tubers, presumably because the extra allowed other sinks for carbohydrate to develop. The growth of some buds of the potato plant is so strongly inhibited that they cannot grow and act as sinks for excess carbohydrate when tubers are removed. Such internal inhibition of growth may sometimes suffice to influence the magnitude of E of normal plants. Removing tubers usually increased sugar and starch content and protein N content of stems and leaves.