We have found that arachidonic (20 : 4) acid is indetectable in phosphatidylinositol and diacyglycerol extracted from Drosophila heads. After careful examinations of the lipid extraction processes and fatty acid detection system (gas-liquid chromatography), we excluded the possibility of the oxidation of polyunsaturated fatty acids or of having overlooked a trace amount of the fatty acid. The precursors of arachidonic, dihomo gamma-linolenic (20 : 3), and gamma-linolenic (18 : 3) acid, were also indetectable in these lipids. On the basis of these results, it appears that the arachidonic acid cascade is essentially absent in Drosophila head, including the brain and compound eyes. Since arachidonic acid is considered to be a key molecule in phosphatidylinositol turnover in the brain, it is of interest that Drosophila brain and eyes do not require arachidonic acid for their functions.