Steady-state mRNA levels of nuclear (rbcS, cab, tubA) and plastid (rbcL, psbA) encoded genes were determined in tomato leaves of different developmental stages. Transcripts were analyzed at four-hour intervals throughout a diurnal cycle in 4 cm-long terminal leaflets, while mRNA levels of the chlorophyll a/b-binding protein (cab), and the small and large subunit of RuBPC/Oase (rbcS, rbcL) are high. At different time points during the day the mRNAs accumulate to characteristic levels. Minor fluctuations of such mRNA levels were determined in the case of rbcS, rbcL, psbA and tubA, while significant alterations are observed for the chlorophyll a/b-binding protein transcript levels. LHCP II transcripts accumulate during the day, reach highest levels at noon and decline to non-detectable levels at 5 a.m. The cab mRNA fluctuates with a periodic length of approximately 24 hours suggesting the existence of a circadian rhythm ("biological clock"), which is involved in gene activation and inactivation. The mRNA oscillation with the same periodic length, but altered amplitude, continues to be present in plants which are kept under extended dark or light conditions. Different mRNA fluctuation patterns are observed for rbcS, rbcL, and psbA under such experimental conditions.