Studies are being carried out to elucidate the factors which regulate the biosynthesis of polyunsaturated fatty acids. The majority of these studies have been carried out using radioactive polyunsaturated fatty acids prepared by total synthesis in our laboratory. In these studies we have measured rates of desaturation and chain elongation of a variety of acids using liver microsomes from rats raised on a balanced as well as on a fat free diet. These studies have shown that different acids are desaturated or chain elongated at different rates. From these studies as well as from in vivo studies we have been able to formulate a hypothesis relative to some of the factors which regulate the production of polyunsaturated fatty acids. These studies have also led us to suggest that microsomes contain more than one chain elongating system and lend support to the developing concept that the microsome contains more than one desaturase.