Lipid-depleted yeast, grown anaerobically, contains only very low amounts of sterols. The hydroxymethylglutaryl-CoA reductase activity, the regulatory enzyme of sterol synthesis in yeast, is also low. Aeration of such cells in a buffer containing a carbon source induces hydroxymethylglutaryl-CoA reductase activity and increases sterol synthesis. The velocity of the increase depends on the carbon source present during the aeration period. Glucose and sugars that are easily converted to glucose were found to be most effective. A supplement of unsaturated fatty acids during anaerobic growth causes a several-fold greater velocity of the enzyme induction and of sterol biosynthesis. Linolenic acid (30 microM) accelerated sterol biosynthesis about 7-fold. Activities of galactokinase and galactose-1-phosphate uridyltransferase, which are involved in the conversion of galactose to glucose, increased several-fold in the supplemented cells within 6 h of aeration, concomitantly with stimulation of sterol synthesis from galactose. It is suggested that the stimulation of enzyme induction and sterol biosynthesis in fatty acid supplemented cells is due to a completion of the protein-synthesizing apparatus during cell growth. A markedly enhanced capacity of these cells to incorporate leucine into acid-precipitable protein supports this assumption.