Intact cells contain a vast number of catalysts that function in morphologically distinct compartments. Communication between these metabolic machineries is mainly accomplished by small molecular cellular constituents such as ions, metabolites and coenzymes. Inorganic orthophosphate in particular seems to play a major role as a rate-limiting factor of carbohydrate metabolism in ascites tumor cells. The protein catalysts called enzymes are usually of a molecular weight about 10,000 and are assembled in functional communities called multienzyme systems. They may be mobile units such as represented by the fermentation pathway, or they may be firmly anchored in structures such as the mitochondria. A representative of a multienzyme system in mitochondria is the Krebs cycle which is responsible for the major respiratory activities of animal cells. Kinetic aspects of enzyme catalysis in multienzyme systems can sometimes be studied in vitro in reconstructed systems which consist of many highly purified enzymes. By variations of the composition of these reconstructed systems in respect to enzymes as well as to coenzymes and ions, it is possible to examine various possibilities of regulatory mechanisms, which control complex cyclic metabolic pathways.