The biological function of ascorbate oxidase (AAO) was not yet clarified, although it was suggested that AAO may be involved in cell growth. We investigated AAO expression and ascorbate metabolism during non-synchronous, synchronous, and elongation cultures of tobacco BY-2 cells. In non-synchronous culture, AAO mRNA was abundant in logarithmic growth phase. Ascorbate content greatly increased during the growth, whereas dehydroascorbate content was slightly increased. In synchronous division culture, AAO mRNA was detected in all phases, but the levels were quite low in G1 phase. Ascorbate content was high in all phases, whereas dehydroascorbate content was low, especially in G1 phase. In elongation culture, the levels of AAO mRNA increased during elongation of the cells. AAO activity in the culture medium, as well as ascorbate and dehydroascorbate contents in the cells, also increased during the elongation. We propose that AAO expression and production of dehydroascorbate are under the control of the cell cycle and that AAO may function apoplastically as an ascorbate oxidizer in the process of cell elongation.