Characteristic steady-state mRNA level oscillations were monitored for the chlorophyll a/b-binding (cab) protein in tomato plants grown under the natural day/night (light/dark) regime as well as under constant environmental conditions. This typical expression pattern was altered when plants were transferred to different light/dark regimes. For example, by shifting the light phase by six hours, a change of the time points of maximum and minimum of expression level was monitored, while the principal oscillation pattern remained the same. It appeared that the transition from dark to light is involved in determining the time points of minima and maxima of mRNA accumulation.After exposing tomato plants to an abnormal light/dark periodicity (e.g. six hours of alternating light/dark) an altered oscillation pattern was determined: within 24 hours two maxima of cab mRNA levels were detected. However, this 'entrained' abnormal rhythm was not manifested at the molecular level and the circadian pattern reappeared under constant environmental conditions (e.g. darkness). This result favours the hypothesis that the oscillation pattern of the cab mRNA in tomato plants is not only endogenous but also hereditary.