By reversing the regimen of environmental lighting a complete inversion of the diurnal rhythm of pineal N-acetyltransferase was brought about. The nocturnal rise of N-acetyltransferase was rapidly reversed by light, yet the diurnal fall of the enzyme was not affected by darkness. Three days’ exposure of the rats to the reversed light regimen was necessary before a 180° inversion was achieved, producing a 10- to 12-fold increase in day-time (dark period) levels of the enzyme. However, after two days a 3-fold increase in the enzyme was already evident in relation to values of control rats kept on a normal lighting regimen. It is concluded that N-acetyltransferase activity and consequently serotonin and melatonin levels of the pineal, although regulated by an endogenous mechanism in the central nervous system, can be shifted by changes in environmental light which may be synchronizing the endogenous rhythms.