Some pathways of hexose-6-phosphate recycling--those involving a breakdown of the hexose skeleton--through carbohydrate metabolism of micro-organisms were analyzed for both metabolic and isotopic effects. Two modes of recycling were proposed based on the degree of alteration of the hexose molecule through the catabolic part of the cycle. Simulated operation of most of these pathways resulted in increased synthesis of hexose-6-phosphate and NADPH, and reduced the NADH and moreover the ATP synthesis within the carbohydrate metabolism. A basic model for the quantitative assessment by means of isotopic studies of the processes of hexose-6-phosphate recycling is presented. The model was initially designed for the study of micro-organisms producing polysaccharides, but it can be extended to other situations.