Alterations in membrane fluidity affect, via an effect on the ease with which proteins may change confonnation, the activity of various enzymes and transport systems. Recent experiments have shown that toxic injury is frequently associated with modijications in physical state and/or lipid composition ofphma membranes. Such modifications are likely to play a role in cell dysfunction, especially in epithelial cells whose optimal function depends on the polarity in membrane fluidity between apical and basolateral domains.