Triacyglycerol content and fatty acid composition has been determined in the liver and in the serum of normal, castrated and testosterone-treated rats. Most variations were found in UNS/SAT ratio, which were referred to an action of the hormone on the delta 5-delta 6-delta 9 desaturases. Results concerning the behaviour of delta 9 desaturase suggest that either different isoenzymes are involved or that a single enzyme undergoes a different regulation according to the substrate involved.