A series of pulses of [3H]thymidine was used to measure cell flow through the mitotic cycle in four regions of the root meristem of Zea. For comparison, colchicine was used to measure cell flow after the supply of pulses of non-radioactive thymidine. Thymidine itself does not, at normal labelling concentrations, disrupt the pattern of cell proliferation within the meristem although it does so in other circumstances. [3H]thymidine, in contrast, does, at labelling concentrations, change the behaviour of the meristem, lengthening cell cycles in the regions normally with rapid cell division and accelerating cell cycles in the quiescent centre. The significance of this disruption is discussed in relation to the use of thymidine in investigating the cell kinetics of meristems.