The determination of membrane fluidity in whole cells of Oenococcus oeni was achieved by membrane probe 1,6-diphenyl-1,3,5-hexatriene fluorescence anisotropy measurements. The results demonstrated instantaneous fluidity variations with cells directly stressed during the measure. Heat (42 degrees C) or acid (pH 3.2) shocks decreased the anisotropy values (fluidising effects), whereas an ethanol shock (10% ethanol, v/v) increased the membrane rigidity. The velocities of fluidity variation with non-adapted or adapted cells (incubation in inhibitory growth conditions) were compared. The adaptation of the cells to acid conditions had no effect on the membrane fluidity variation after an acid shock. In contrast, the rates of membrane fluidity variation of adapted cells were 5- and 3-fold lower after a heat shock and an ethanol shock, respectively. These results suggest the positive effect of an adaptation on the membrane response and can help to explain the mechanisms of stress tolerance in Oenococcus oeni.