The kinetics of proliferating cells in sub-populations throughout the root meristem of Avena sativa were studied by means of labelling-techniques and cell-flow experiments. Estimates of the mean duration of the cell cycle and its constituent phases G,, S, G2 and M were derived from the ‘fraction of labelled mitoses’ curves. Progressive reduction in cell cycle duration, largely brought about by a decrease in G,, were found in cortex and stele with increasing distance from the tip. Proliferation fractions, calculated from the ratio: duration of cell cycle/cell-doubling time, reveal that most of the sub-populations exclusively consist of cells in the cell-division cycle. The reduction in proliferation fraction found in the proximal sub-population of the meristematic cortex points to a substantial proportion of non-proliferating cells due to the cessation of proliferative activity at the margin of the meristem before it begins to differentiate. We concluded from continuous-labelling experiments that all cells in the quiescent centre progressed through the cell cycle, which accords with the high proliferation fraction predicted from radial transit rates of cap initials by Harkes (1976).