In normolipidaemic young rats the intraperitoneal administration of diazepam (10 mg/kg) induced a marked fall of the total serum lipids and total cholesterol. A dose of 5 mg/kg brought about an increase of HDL2 lipoprotein fraction and an elevation of the HDL2/HDL2.HDL3 ratio. Margarine administration elicited a significant augmentation of the total serum lipids and a reduction of the HDL2/HDL2.HDL3 ratio. The intraperitoneal injection of diazepam in margarine-treated rats brought about a decrease of total serum lipids and total cholesterol, (when compared to the margarine values), an obvious elevation of the HDL2 fraction and of HDL2/HDL2.HDL3 ratio. The LDL1 (beta) fraction was decreased. These data confirm our earlier findings which show that diazepam improves the HDL cholesterol/total cholesterol ratio.