The importance of aquaporin expression in water permeability in Saccharomyces cerevisiae was assessed by measuring the osmotic water permeability coefficient (P-f) and the activation energies (E-a) from both hypo- and hypertonic experiments performed with whole protoplasts from four strains differing in aquaporin level of expression: parental, double-deleted and overexpressing AQY1 or AQY2. Double-deleted (lower Pf) and AQY1-overexpressing strains (higher Pf) presented linear Arrhenius plots with E,, consistent with fluxes mainly through the lipids [16(.)3 kcal mol(-1) (68(.)2 kJ mol(-1))] and with a strong contribution of channels [9(.)6 kcal mol(-1) (40(.)2 kJ mol(-1))], respectively. The Arrhenius plots for the parental (swelling experiments) and overexpressing AQY2 strains (swelling and shrinking experiments) were not linear, presenting a break point with a change in slope around 23 degrees C. The Ea values for these strains, calculated for temperatures ranging from 7 to 23 degrees C, were lower [9-5 kcal mol(-1) (39(.)7 kJ mol(-1))] than the values obtained from 23 to 38 degrees C [17 kcal mol(-1) (71(.)-1 kJ mol(-1))]. This behaviour indicates that only in the lower temperature range did the water fluxes occur predominantly via the water channels. The permeabilities for each strain relative to the deletion strain show that an increase in permeability due to the presence of aquaporins was more relevant at low temperatures. Following our results, we propose that water channels play an important role for osmotic adjustment of yeast cells at low temperature.