Hepatic polysome profiles vary in untreated rats as a function of time of day. The ratio of polysomes to total ribosomes increases from 50 to 73 per cent in darkness. There is also a daily rhythm in tyrosine transaminase activity which resembles but does not coincide with the polysome rhythm. Both rhythms are dependent on the cyclic ingestion of dietary protein, and disappear in rats given a protein-free diet.