The effect of a controlled lighting schedule on the activity of a weanling rat liver extract that stimulates DNA synthesis in regenerating adult rat liver, and on the response of the test animals to the extract, has been investigated. Both activity of the extract and endogenous DNA synthesis in the weanling animals follow the same distinct diurnal rhythm. Reversal of the lighting schedule reverses the rhythm of endogenous DNA synthesis but activity of the extract no longer correlates with the peak of DNA synthesis. Diurnal rhythm also has a striking effect on DNA synthesis in the regenerating test animal, but the extract increases DNA synthesis to the same relative degree, regardless of the time of day the hepatectomy is performed.