Mitosis in root tips of Zea mays is partially synchronized by 12- or 24-hour treatments in 5AU and peaks in the mitotic index occur at about 14, 22, and 30 hours after removal of the substance. The cap initials are more sensitive to the 5AU than the meristematic parts of the stele and their mitotic index hardly reaches the normal value even at the peaks. This may be correlated with the absence of a G1phase from the normal mitotic cycle of the cap initials and with their very high normal rate of division. There is a general slight rise in the mitotic index in the quiescent centre probably the result of injury by the 5AU to the surrounding cells. Differences in the pattern of synchronization between the 12- and 24- hour-treated roots suggest that the 5AU retards the passage of the cells through the mitotic cycle as well as stopping it in the DNA-synthetic phase.