An apparatus for measuring the rates of photosynthesis of leaves is described. In principle, the method consisted of interrupting the flow of air over a leaf for 15sec, during which time air containing 14C dioxide was passed over it. The amount of14C assimilated by the leaf was then measured. The results were compared with those obtained using an infra-red gas analyser. The principal cause of the discrepancies between the results appeared to be preferential fixation of 12C by the leaves and loss of respiratory 12C at low light intensities. There was a linear relation between the results obtained by the two methods and thus, using the 14C method, the rates of net photosynthesis could be estimated. By fitting a set of neutral-density filters over the chamber that enclosed the leaf, the rates of net photosynthesis were determined for several light intensities simultaneously. Examples are given of curves showing the relation between light intensity and net photosynthesis for leaves of Beta and Phaseolus plants.