Sublethal heat and ethanol exposure induce essentially identical stress responses in yeast. These responses are characterized by the induction of heat shock proteins, proteins requiring a temperature above about 35 degrees C or ethanol levels above a threshold level of 4-6% (v/v) for strong induction. One induced protein, Hsp104, contributes to both thermotolerance and ethanol tolerance, while others are anti-oxidant enzymes. Heat and ethanol stress cause similar changes to plasma membrane protein composition, reducing the levels of plasma membrane H(+)-ATPase protein and inducing the plasma membrane-associated Hsp30. Both stresses also stimulate the activity of the fraction of H(+)-ATPase remaining in the plasma membrane. The resulting enhancement to catalysed proton efflux from the cell represents a considerable energy demand, yet may help to counteract the adverse effects for homeostasis of the increased membrane permeability that results from stress.