Isolated leaf disks are illuminated under constant conditions in a specially arranged chamber and their increase in dry weight within a certain time is taken as a measure of photosynthetic intensity. The weight increment is estimated by comparing the dry weight of the illuminated disks with that of control disks sampled in the close vicinity of the former and dried immediately after sampling. Disks are taken from the leaf blades following a pattern which allows the average original weight of the control and experimental disk sets to be taken as identical within ±0·5%. During illumination in the assimilation chamber the disks are inserted into circular openings bored in agar plates; thus, they remain satisfactorily turgescent throughout the experiment, since they take up water through the section area from the gel. Air from the chamber circulates through a heat exchanger and the concentration of CO2 is maintained constant in the atmosphere. The chamber is illuminated from above by a set of light bulbs producing an illumination intensity of 1·85×105 erg/cm2 sec. It was shown that the dry weight of the disks in the chamber increases proportionally with time for several hours. During this time the effect of injury caused through sampling as well as the conditions to which the disks are exposed in the assimilation chamber, apparently do not interfere with intense photosynthesis. Parallel samples from the same leaves give the same values of dry weight increase; the rate of photosynthesis (dry weight increments) estimated by this method, is thus characteristic for the leaves investigated.