In an attempt to understand the metabolism by the liver of fatty acids (FA) of different chain length, we have studied the incorporation of [1- 14C]-labeled C 2, C 8, C 10, C 12, and C 16 into cellular lipids by HepG-2 cells. Over 90% of the radiolabeled FA were detected in phospholipids (PL) and triacylglycerols (TAG). The incorporation of C 12 and C 16 was three to four times higher than that of C 8 and C 10 (and reached 35 nmoles per mg protein after 1.5 h). The radioactivity of C 2, C 8, and C 10 was recovered mainly in PL. C 12 and C 16 were incorporated at approximately equal amounts into PL and TAG. The radioactivity of both C 2 and C 8 was recovered exclusively in long-chain FA, suggesting oxidation of C 8 into C 2 units prior to FA synthesis. C 10 likewise yielded mainly long-chain FA. However 10% of unchanged C 10 was found in PL and up to 30% in TAG. 14C- C 12 was largely incorporated unchanged. Under these conditions, the presence of C 10 and C 12 in PL and TAG was shown also by gas liquid chromatography. In the presence of either C 2, C 8, or C 10, up to 30% of 14C-monounsaturated FA were detected in PL and TAG. With C 12 and C 16, the fraction of 14C-monounsaturated FA was much smaller suggesting that extensive desaturation occurred during de novo synthesis.