Root tips of Vicia faba and Zea mays have been subjected to continuous irradiation from radium or cobalt-6o for long periods at various dose rates. The rates of mitosis and the damage to the chromosomes (assessed as percentages of cells with micronuclei) have been measured in four regions of the meristems. In Zea rates of mitosis are reduced under chronic irradiation, except in the quiescent centre, and the cap initials are particularly sensitive. In Vicia the main change in mitosis is that the quiescent centre increases its rate, but at 12° C there is a slight stimulation of division all over the meristem. In Vicia increasing the dose rate or lowering the temperature increases the nuclear damage. At 190 C there is an increase in damage with accumulated dose per cell cycle, but the data at 120 C do not fall on the 190 C curve, suggesting that there may be a temperature effect on damage other than that caused by changing the duration of the cell cycle. The differences in radiosensitivity between the different regions of the meristem are due solely to differences in the rates of mitosis of the cells.