Suspensions of liver cells prepared from young female rats and kept under a light‐dark cycle show a circadian rhythm in tyrosine: 2‐oxoglutarate aminotransferase activity. The rhythmicity, which is obviously not homologous to that one observed in the whole animal, persists for some days under constant illumination. In the course of about two weeks, however, the rhythm is damped out. After 3 weeks under constant conditions, rhythmicity can be induced again by a one‐hour period of darkness. In the arhythmic cultures, average enzyme activity is much lower than in the rhythmic cultures. The transition from the non‐oscillatory to the oscillatory state is accompanied by an increase in TAT synthesis, which can be blocked by cycloheximide and partially by α‐amanitine.