This investigation was performed to determine whether acute ethanol administration (5 g and 3 g/kg body wt.) alters the lipid composition both of liver plasma membrane (LPM) and serum in rat or not. The changes of lipid composition of LPM and serum were observed during peak values which blood ethanol concentration was achieved, after both 5 g and 3 g/kg body wt. ethanol administration; LPM cholesterol (CH) content was significantly decreased, but LPM phospholipids (PL) content was not changed statistically. Serum CH and PL contents were significantly increased. However, CH/PL molar ratio showed significant decreases in both LPM and serum. SPM/PC in LPM was also decreased significantly. These changes of lipid composition in LPM were associated with the increase of the fluidity as demonstrated in our previous reports. These lipid values in ethanol treated animals expressed as a ratio of those from corresponding control animals, taken as 1, consisted with 0.7-0.8, that is very interesting from viewpoint of "homeoviscous adaptation", and in addition, suggesting the possibility that serum CH/PL molar ratio may be useful for the estimation of the changes of lipid composition concerned with LPM fluidity.