When chemostat cultures of Saccharomyces cerevisiae CBS 8066 and Candida utilis CBS 621, grown under glucose limitation, were pulsed with excess glucose, both organisms initially exhibited similar rates of glucose and oxygen consumption. However, striking differences were apparent between the two yeasts with respect to the production of cell mass in the culture and metabolite excretion. Upon transition from glucose limitation to glucose excess, S. cerevisiae produced much ethanol but the growth rate remained close to that under glucose limitation. C. utilis, on the other hand, produced little ethanol and immediately started to accumulate cell mass at a high rate. This high production rate of cell mass was probably due to synthesis of reserve material and not caused by a high rate of protein synthesis. Upon a glucose pulse both yeasts excreted pyruvate. In contrast to C. utilis, S. cerevisiae also excreted various tricarboxylic acid cycle intermediates, both under steady-state conditions and after exposure to glucose excess. These results and those of theoretical calculations on ATP flows support the hypothesis that the ethanol production as a consequence of pyruvate accumulation in S. cerevisiae, occurring upon transition from glucose limitation to glucose excess, is caused by a limited capacity of assimilatory pathways.