Mice maintained for 48 d on a diet containing 10% partially hydrogenated coconut oil (PHCO) slept more than twice as long as mice maintained on a standard, laboratory mouse chow (CHOW) diet when injected i.p. with pentobarbital. At this time, the liver of mice maintained on the PHCO diet exhibited a decrease in the mol% of phosphatidylethanolamine and an increase in phosphatidylcholine and phosphatidylinositol compared to the liver of control animals. In the brain, no changes in phospholipid composition were detected. However, in both brain and liver, alterations in the fatty acid composition of the phospholipid classes were observed, with eicosatrienoic acid (20:3, n-9) increasing and the polyunsaturates, particularly docosahexaenoic acid (22:6, n-3), decreasing significantly. These changes in lipids of the brain and liver may be related to the prolonged sleeping time of the animals on the PHCO diet by altering the metabolism of pentobarbital in the liver and/or altering the sensitivity of the brain membranes to the presence of the drug.