A transference chamber was developed to measure the osmotic water permeability coefficient (P os) in protoplasts 40 to 120 µm in diameter. The protoplast was held by a micropipette and submitted to a steep osmotic gradient created in the transference chamber. P os was derived from the changes in protoplast dimensions, as measured using a light microscope. Permeabilities were in the range 1 to 1000 µm s-1 for the various types of protoplasts tested. The precision for P os was =40%, and within this limit, no asymmetry in the water fluxes was observed. Measurements on protoplasts isolated from 2- to 5-d-old roots revealed a dramatic increase inP os during root development. A shift inP os from 10 to 500 µm s-1occurred within less than 48 h. This phenomenon was found in maize (Zea mays), wheat (Triticum aestivum), and rape (Brassica napus) roots. These results show that early developmental processes modify water-transport properties of the plasma membrane, and that the transference chamber is adapted to the study of water-transport mechanisms in native membranes.