The rates of synthesis of beta-galactosidase (EC 22.214.171.124) and the intracellular levels of cyclic 3',5'-adenosine monophosphate (cAMP) soon after the addition of glucose or glycerol to exponentially growing cultures of Escherichia coli have been determined. Within 10 s of its addition, glucose, but not glycerol, lowered the apparent initiation frequency of lac messenger ribonucleic acid. The glucose-generated reduction in initiations is identified as catabolite repression by its reversibility with cAMP. The intracellular cAMP levels respond virtually identically to glucose and glycerol additions. Thus, no correlation was observed between the rate of messenger ribonucleic acid initiation and the level of cAMP.