A protocol based on radio gas chromatography demonstrates that when HeLa cells are deprived of valine for short periods of time (6-7.5 h), their overall fatty acid biosynthetic activity is depressed after a latency of a few hours. The transfer of newly synthesized fatty acyl units to phospholipids is curtailed much faster than their transfer to triacyglycerols. Despite the cut-back in fatty acid biosynthesis, valine deprivation causes a lipid accumulation in the cells. Valine deprivation appears to affect de novo synthesis of fatty acid units from acetate more rapidly than desaturation and elongation. When valine is returned to the valine-deprived culture, overall fatty acid biosynthesis is resumed well within 2 h. Newly synthesized fatty acyl units are transferred to both the phospholipids and the 1,2-diacylglycerols of the cells but not initially to the triacylglycerols.