Summary: Anaerobiosis, dinitrophenol and sodium azide decreased catabolite repression of β-galactosidase caused by glucose in non-growing Escherichia coli k-12. Anaerobiosis had no effect on the efficiency of exogenous cyclic AMP in stimulating enzyme synthesis. In contrast, dinitrophenol and sodium azide made the synthesis of β-galactosidase more responsive to exogenous cyclic AMP because the effect of cyclic AMP with dinitrophenol or with sodium azide was markedly greater than the sum of the separate effects of cyclic AMP and of each of the uncouplers. This effect of the uncouplers was unrelated to their action on oxidative phosphorylation because it still occurred under anaerobic conditions. It was also not due to increased accumulation of cyclic AMP by enhanced uptake or by inhibition of cyclic AMP-phosphodiesterase.