Data are presented demonstrating that the presence in vivo of adenosine 3',5'-monophosphate (3',5'-AMP) causes a rapid depletion of glycogen storage material in the cellular slime mold. The effect of adenosine 5'-monophosphate (5'-AMP) is twofold, stimulating both glycogen degradation and synthesis. In pseudoplasmodia, cell-free extracts appear to contain at least two species of glycogen phosphorylase, one of which is severely inhibited by glucose-1-phosphate and another which is only partially inhibited by this hexose-phosphate. In some cases, 5'-AMP partially overcomes the inhibition by glucose-1-phosphate. Data presented here also indicate the existence of two forms of glycogen synthetase, the total activity of which does not change during 10 hr of differentiation from aggregation to culmination. During this period there is a quantitative conversion of glucose-6-phosphate-independent enzyme activity to glucose-6-phosphate-dependent activity. It is suggested that one effect of 3',5'-AMP is closely related to enzymatic processes involved in the rapid conversion of glycogen to cell wall material and other end products accumulating during sorocarp construction.