The effect of sterols on the rates of nonelectrolyte movement in multilayered liposomal membranes formed from various phosphatidylcholine (PC) analogues was studied. We measured the temperature dependence of the permeation of small, polar solutes through lipid bilayers in the liquidcrystalline state of the following saturated phosphatidylcholines: diesters containing 14, 16, or 18 carbon atoms in each acyl chain; the corresponding diethers, which lack the carbonyl groups; and alkyl analogues, such as 2-octadecyleicosylphosphorylcholine (OEPC), which have no diacylglycerol or glycerol diether moiety. In addition, a diether-PC having one saturated and one unsaturated chain was used, and analogues of OEPC were synthesized in which the choline group is modified. At temperatures above the gel to liquid-crystalline phase transition of these diester, diether, and alkyl analogues of PC, the 30-hydroxysteroids cholesterol and ergosterol decreased the permeability of glycerol, urea, acetamide, and glucose, whereas epicholesterol and lanosterol exerted little effect on the reflection coefficient for acetamide penetration. Cholesterol reduced solute permeability in liposomes from OEPC analogues having increased separation

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