A method is described for the determination of total fatty acids in small amounts of blood plasma. The plasma lipids are extracted in an alcohol–acetone mixture. Following saponification and hydrolysis, the fatty acids from an aliquot of their benzene solution are dissolved in alcoholic thymol blue and titrated with tetramethyl ammonium hydroxide as recommended by Schmidt–Nielsen. The method is applicable to the study of alimentary hyperlipemia in rats.