The question of posttranscriptional control during cumulative repression of Escherichia coli carbamoylphosphate synthase has been examined by following the kinetics of repression and by comparing messenger ribonucleic acid and enzyme levels after growth under various conditions. The data provide no evidence for control of synthesis at a level other than transcription. They suggest, however, that enzyme inactivation (or turnover) plays a significant role in the establishment of repressive conditions.