The yeast Saccharomyces cerevisiae was grown on 10% glucose medium and subsequently transferred to fresh medium containing 2- and 3-carbon substrates. Under these conditions, the yeast rapidly acquired an oxidative capacity, as evidenced by oxygen uptake rates and 14CO2 evolution rates during respiration on ethanol or (14C)acetate. The assimilative capacity for 2-carbon substrates developed more slowly and followed the induction of isocitrate lyase. Washed yeast transferred to the basic medium containing no added carbon substrate possessed only low levels of isocitrate lyase after a 6-h adaptation. After 6 h, isocitrate lyase was present at high levels in cells transferred to a range of ethanol concentrations but was present in only low amounts in cells transferred to acetate. The role of ethanol as an inducer of isocitrate lyase is discussed.