In bioluminescent bacteria growing in shake flasks, the enzyme luciferase has been shown to be synthesized in a relatively short burst during the period of exponential growth. The luciferase gene appears to be completely inactive in a freshly inoculated culture; the pulse of preferential luciferase synthesis which occurs later is the consequence of its activation at the level of deoxyribonucleic acid transcription which is attributed to an effect of a "conditioning" of the medium by the growing of cells. Although cells grown in a minimal medium also exhibit a similar burst of synthesis of the luminescent system, the amount of synthesis is quantitatively less, relative to cell mass. Under such conditions, added arginine results in a striking stimulation of bioluminescence. This is attributed to a stimulation of existing patterns of synthesis and not to induction or derepression per se.