Different fractions of ribosomal RNA (28 S, 18 S, 5 S) and transfer RNA (4 S) of rat liver were separated using phenol extraction and gel electrophoretic methods. The concentrations of all fractions except 5 S show a daily rhythmicity with a maximum between 11.00 h and 19.00 h and a minimum at 7.00 h.3H‐uridine incorporation also varies within 24 h; in the 4 S fraction two maxima at 11.00 h and 23.00 h and two minima at 19.00 h and 7.00 hcan be distinguished, whereas in the 28 Sand 18 S one maximum about 7.00–11.00 h and one minimum at 23.00 h is observed. Injection of cycloheximide 2.5 h before the RNA measurements reduces the amount and3H‐labelling of the 28 S and 18 S fractions considerably.