Male Wistar rats with an initial weight of 170g were maintained on a nutritionally adequate diet, and diazepam was administered at a dose of 10 mg/kg/d. Control animals were pair-fed an adequate diet. The feeding was continued for 180 d, and the effects on brain lipid contents were studied. It was found that the contents of the phospholipids, phosphatidylethanolamine and phosphatidylserine, the monogalactosyl glycolipids, hydroxy and nonhydroxy fatty acyl galactocerebroside, sulfoglycolipids, and the gangliosides, GM1, GD1a, GD1b, and GT1b, were significantly reduced in the brain of diazepam-treated rats. There was a significantly increased content of phosphatidylinositol after 180 d of diazepam treatment. The results suggest that changes in brain lipid content may mediate the adaptive changes that occur upon prolonged exposure to diazepam.