Three different responses of dividing root-meristem cells to cycloheximide have been observed. Cells in anaphase or telophase can complete these stages and pass into G1; movement of prophase and metaphase cells into anaphase is reversibly inhibited; entry of G2 cells into mitosis is irreversibly inhibited. While this last effect is probably a consequence of inhibition of protein synthesis by cycloheximide, it is suggested that the reversible mitotic arrest induced by the drug results from inhibition of some other cellular activity, possible related to energy transfer.