The rates of photosynthesis and respiration in Potamogeton crispus, Nerium indicum, Camellia japonica, and Iris pseudoacorus were measured by a simple volumetric method devised by the author. The apparatus consists of two glass tubes, a and c, as shown in Fig. 1. In tube a a leaf strip of Potamogeton was inserted and filled with 0.1M carbonate-bicarbonate CO2-buffer solution. In the case of land plant, a leaf strip and filter paper strips soaked with CO2-buffer solution of 0.1-1.0M were inserted in tube a and separated by a perforated separator. Movement of a bubble in tube c corresponding to O2 evolution under illumination and O2 consumption in darkness was measured. Measurements were made simultaneously with six volumeters, one of which was used as a thermo-barometer. The distinctive features of this method are: 1) Apparatus and manipulation are very simple and measurement can be done in 10minutes with readings made at 2 minute intervals. 2) Air temperature presents no disturbance. 3) One graduation of the micrometer is equivalent to 0.03μl of gas. By using a finer capillary and larger leaf strips more detailed measurement can be made. 4) As many strips can be prepared from a leaf, individual variation may be restricted to a small level. 5) Gas exchange is mainly carried out through the cut surface of leaf strips rather than through their stomata. Therefore, the effect of stomatal aperture on the rate of gas metabolism is small.