The decrease rate of the water potential was found to have a great effect on yeasts submitted to hypertonic shifts. The application of slow and linear decreases of the water potential of the medium to cells of Saccharomyces cerevisiae demonstrated that the cells could survive (90 to 100% of viability) very low levels of water potential (Psi = -1O1 MPa). Resistance of the cells was examined through viability measurements and cell volume changes. The kinetics of cell volume variation were measured continuously using an on-line image analysis system coupled to a microscope. No biological response of the cells occurred, due to the lack of a usable carbon-energy source in the medium. The viability level was found to be a function of the water exit flow rate from the cells. The denaturation of the membrane structure was assumed to be involved in such phenomena.