A method is described for the simultaneous determination of rapid changes of the cell turgor pressure (hydrostatic pressure) in algal cells (cell size must be at least 3 mm in diameter), and of the net volume flow across the cell membrane arising after a change of the cell turgor pressure or of the osmotic pressure in the outside medium. On the basis of the equations of irreversible thermodynamics it is possible to calculate the hydraulic conductivity of the cell membrane from these measurements, as it is theoretically shown. The hydraulic conductivities of the marine alga Valonia utricularis determined in two independent ways (by osmotic and hydrostatic experiments) are equal. For exosmosis, Lpex (hydrostatic) and Lpex (osmotic) amounted to (9,6 ± 1,0) ·10-7 and (9,8 ± 1,9) · 10-7 respectively cm · sec-1 · atm-1, and for endomosis, Lpen (hydrostatic) was (9,4 ± 1,1) ·10-7 cm · sec-1 · atm-1. A polarity in the water movement across the cell membranes as discussed in the literature could not be found for Valonia utricularis.