Cells of Escherichia coli grown on maltose can be induced by the addition of thiomethyl galactoside to produce β-galactosidase. If cells are irradiated shortly after induction, the transcription of the DNA ceases, and the enzyme produced by the messenger RNA is observed to reach a maximum. From these data the calculated half-life of unstable messenger RNA is given over a temperature range from 8.1 minutes at 10°C to 0.7 minute at 45°C. The kinetics of cessation of transcription give information on both messenger RNA decay and rate of transcription. Arrhenius graphs for both these rates are given, and the activation energies measured are 11,000 calories per mole for decay and 22,000 calories per mole for transcription. This relation to temperature is characteristic of enzymatic behavior.