The technique of metabolic flux analysis was implemented to elucidate the flux balancing of Saccharomyces cerevisiae cultivated in a multistage continuous stirred tank reactor fermentation environment. The results showed that the majority of the substrate (97.70 +/- 0.49%) was funneled into the glycolytic pathway, while the remainder was subdivided between the pentose phosphate pathway and pathways for polysaccharide synthesis. At the pyruvate node, 87.30 +/- 1.38% of the flux was channeled through the reaction governed by pyruvate decarboxylase. Fluxes through the pyruvate dehydrogenase bypass were maintained at a constant level (82.65 +/- 1.47%) irrespective of the configuration of the fermentation setup. Activity through the TCA "cycle" was replenished by the reaction catalyzed by pyruvate carboxylase and by the transport of cytosolic oxaloacetate across the mitochondrial membrane. The CO(2) evolution rate varied as fermentation progressed; however, the yield coefficient of CO(2) remained at a constant value. Although a constant yield of ethanol (0.42 g of ethanol/g of glucose) was obtained, operations of the TCA cycle were gradually switched from partially reductive to partially oxidative pathways from the first fermenter to the fourth fermenter.