During the starvation-induced spherulation of the true slime mold Physarum polycephalum, a metabolic shift occurs from glycogen synthesis during growth to slime (polygalactose) secretion during this differentiation. The synthesis rate of glycogen during growth is 3.5 spec. mU, of slime during spherulation 2.1 spec. mU. The specific activity of UDPGpyrophosphorylase (EC 2.7.7.9) increases during the first nine h of starvation by about 40% and then decreases. After 24 h of starvation only 30% of the specific activity present during growth is left. During differentiation induced by 0.5 mannitol its specific activity decreases continuously, without, an initial increase, to about 25% of the initial value. The specific activity of UDPGpyrophosphorylase is not influenced by the addition of cycloheximide or actinomycin-C during the differentiation.