The swelling of isolated chloroplasts of spinach and poke leaves in sucrose solutions of different osmotic pressures was studied.The degree of swelling was estimated by various methods: water content determined by gravimetric method, the volume measured with a hematocrit, the size distribution observed by light microscopy, and the ratio between optical densities at 540 and 435mμ All these four methods were found to give results in good agreement.A wide diversity of behaviour towards variation of the osmotic pressure of suspending medium was exhibited by the chloroplasts. Within the concentration range of approximately 0.3–0.5 M scarcely any volume change was found, probably owing to osmotic regulation by the chloroplasts. Once the swelling takes place, however, it is an irreversible process.The swelling decreased with increasing acidity of solution.