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Storage of glycogen in yeast (Saccharomyces carlsbergensis) cells was strongly suppressed by the presence of nitrogen sources. Peptone initiated glycogen breakdown within minutes. This effect could not be duplicated with ammonium ions alone nor with single amino acids or mixtures of a few amino acids, but it could be duplicated by the addition of all the amino acids in the molar ratios found in casein. If fewer amino acids were supplied, glycogen was initially synthesized but then depleted after 30-60 min at 28 "C, the time being shorter for more easily utilized amino acids. In the presence of glucose, dinitrophenol ( M) slowed down or inhibited glycogen synthesis, but it did not initiate glycogen breakdown in the absence of glucose. Energy charge and the concentrations of adenylates and UDP-glucose were not significantly different in the presence and absence of peptone; pyruvate accumulated under the former condition. No turnover of glycogen was observed during glycogen synthesis, pointing to an effective regulation of glycogen metabolism via an unknown mechanism.