AbstractThe biochemical information now available on the cellular slime mold is sufficient to attempt a kinetic model of a portion of the metabolism essential to differentiation. A computer model has been constructed which is consistent with thein vivoandin vitrodata obtained for this system, and which makes specific predictions now being tested in the laboratory.Uridine diphosphoglucose (UDPG) is a precursor of soluble glycogen as well as of the saccharide end products of differentiation, such as cell wall material. The rate of turnover of UDPGin vivois known, and is sufficient to account for the observed accumulation of end products over the period of time normally required during differentiation. The accumulation patterns during development of the various polysaccharides and of UDPG, glucose‐1‐phosphate (G‐1‐P), and uridine triphosphate (UTP) are known.Kmvalues of UDPG synthetase for G‐1‐P and UTP have been determined, as have changes in specific enzyme activity during development. A series of differential equations describing the synthesis and utilization of UDPG have been used to construct a computer model for the conversion of glycogen through G‐1‐P and UDPG to the end products of differentiation. An analysis of this model demonstrates that an increase in UDPG pyrophosphorylase concentrationin vivocannot account for the enhanced rate of synthesis of UDPG nor for the accumulation patterns observed. The major controlling factor in this system is shown to be the availability of G‐1‐P. This analysis reflects upon: (1) the general significance of alterations in the concentration of enzymes during differentiation, and (2) the importance of understanding control mechanisms responsible for the availabilityin vivoof precursors for synthetic pathways necessa