Changes in the amount of ribulose 1,5-bisphosphate carboxylase/oxygenase (Rubisco; EC 4·1·1·39) synthesized and degraded and the levels of rbcL and rbcS mRNAs were examined in the eighth leaf blades of rice from emergence to senescence. Synthesis of Rubisco was very active during leaf expansion, became quite low at the time of full expansion and then declined further during senescence. The changes in the levels of rbcL and rbcS mRNAs co-ordinated approximately with those in the amount of Rubisco synthesized. Thus, it is suggested that the amount of Rubisco synthesized is determined primarily by the levels of rbcL and rbcS mRNAs during the life span of the leaves. Degradation of Rubisco started just before the time of full expansion and became far more active than its synthesis during senescence. Since the synthesis of Rubisco during senescence scarcely contributed to its amount, it can be concluded that the degradation of Rubisco is the major determinant for the amount of Rubisco in senescent leaves. The decline in the level of rbcL mRNA occurred much earlier in the developmental stage and proceeded at a much faster rate than that of rbcL DNA, indicating that the level of rbcL DNA is not a major determinant for the level of rbcL mRNA in senescent leaves of rice.