Abstract—The effect of sleep deprivation on thein vivoandin vitrotritiated amino acid incorporation into brain proteins was studied in the rat at three age levels. Sleep deprivation was induced either by water tank or handling methods. Three experimental groups of animals were used: control, sleep deprived and post deprivation sleeping rats.A significant decrease of protein synthesis was found in the cerebellum, telencephalon and in crude subcellular fractions of brainstem of adult rats selectively deprived of paradoxical sleep. However, no alteration of protein synthesis was observed eitherin vivoorin vitro, in the same brain regions or in the liver after the rebound of paradoxical sleep following deprivation.In four crude subcellular protein fractions a specific increase of thein vitrolabelled amino acid incorporation was observed in the brain stem of 24‐day‐old rats allowed to recuperate after sleep deprivation as compared with the deprived rats. No significant changes were seen in the telencephalon.No alteration of incorporation was found in 7‐day‐old rats deprived of sleep.The possible functional significance of these results is discussed in relation to stress and to variations in the size of the precursor pool for protein