The main interest of the present study was to determine possible alterations in fatty acid composition in rat plasma and liver phospholipids (PL) caused by chronic ethanol consumption, diazepam treatment and chronic consumption of alcohol and diazepam together. Chronic ethanol consumption (11 g/kg/d) elevated the proportion of plasma saturated (SFA) and mono-unsaturated fatty acids (MUFA), and decreased the most important polyunsaturated fatty acids (PUFA) in plasma and liver PL. The alterations in fatty acid composition in liver PL indicate that ethanol can change the composition of cell membrane lipids. Diazepam treatment (10 mg/kg/d) elevated the contents of SFA and MUFA in plasma PL. On the other hand, diazepam produced a drastic decrease in 22:6n-3 docosahexaenoic acid (DHA) in plasma and liver PL. These changes in plasma PL fatty acid composition indicated a disturbance of fatty acid metabolism. The changes in fatty acid contents of plasma and liver PL were the greatest in rats treated with a combination of ethanol and diazepam, in which there was a summation of the effects of ethanol or diazepam alone, and the effects were intensified by an ethanol-diazepam interaction.