Fatty acid elongases and desaturases act in concert in modifying the fatty acid acyl chain in respect of length and degree of unsaturation, respectively. The initial and rate-controlling condensation reaction of very-long-chain fatty acid (VLCFA) elongation is catalysed by the elongase enzymes referred to as elongation of very-long-chain fatty acids (ELOVLs). The desaturases introduce a double bond at a specific position on the acyl chain of fatty acids and can be divided into two distinct families referred to as stearoyl-CoA desaturases (SCDs), and fatty acid desaturases (FADS). Both the desaturases and elongases display distinct fatty acid substrate specificity depending on the chain length and degree of unsaturation. Of the elongases; ELOVL1, ELOVL3, ELOVL6 and ELOVL7 prefer saturated and monounsaturated fatty acids as substrate; and ELOVL2, ELOVL4 and ELOVL5 are selective for polyunsaturated fatty acids (PUFAs). Likewise, SCDs prefer saturated fatty acids while FADS favour PUFAs. Although the general function of the Elovl, Scd, and Fads, genes is known, the physiological consequence of the tissue-specific demand for VLCFAs and PUFAs produced by the different elongases is not fully understood. In order to understand their regulation and physiological role, we have investigated the expression of both elongases and desaturases in mouse liver and BAT, in respect of diurnal variation, temperature dependent regulation and nutritional influence.