A comparative study was made on the fate of linoleic, arachidonic, and docosa-7,10,13,16-tetraenoic acids in various subcellular fractions of liver and testis from rats of different ages. It was demonstrated that testicular microsomes can desaturate and elongate linoleic and arachidonic acids in a manner similar to liver microsomes, and that testicular mitochondria can convert docosa-7,10,13,16-tetraenoic acid to arachidonic acid. Testicular or liver microsomes actively desaturate linoleic acid to gamma-linolenic acid and eicosa-8,11,14-trienoic acid to arachidonic acid. However, it was impossible to measure in vitro any direct conversion of adrenic acid (22:4 [n - 6]) to docosapentaenoic acid (22:5 [n - 6]) by either liver or testicular microsomes. Docosa-7,10,13,16-tetraenoic acid is incorporated preferentially into the triglyceride fraction of total testis, mitochondria, and microsomes, while linoleic and arachidonic acids are incorporated more into phospholipids. The capacity of testicular microsomes, but not of liver microsomes, to synthesize polyunsaturated fatty acids declines with age. It is proposed that the synthesis of acids of the linoleic family proceeds in two stages, a rapid one in which arachidonic acid is made and a second, slower, one in which C(22) and C(24) acids are synthesized. In addition, there appears to be a cycle between microsomes and mitochondria that acts to conserve essential polyunsaturated C(20) and C(22) fatty acids by means of synthesis and partial degradation, respectively. This cycle would restrict the loss of essential fatty acids and might be of importance for the supply of arachidonic acid in testis under specific requirements and especially in older animals.