Repressed respiration of Escherichia coli cells grown in the presence of 2% glucose was derepressed when the cells were incubated in a buffer containing casamino acids. The glucose-repressed cells were deficient in succinate dehydrogenase [EC 1. 3. 99. 1] and isocitrate lyase [EC 4. 1. 3. 1] activities, which increased during the incubation. The increases in respiratory activity and enzyme activity on incubation were repressed by glucose, but except for isocitrate lyase these repressions could be restored by the addition of cyclic adenosine 3', 5'-monophosphate. Inhibitors of protein synthesis blocked the increase of enzyme activity on incubation without glucose, or with glucose and the cyclic nucleotide.