The effects of local tumour radiation over the dose range 7.5-30 Gy on the growth and cell kinetics of rat hepatoma H-4-II-E have been investigated. A plot of growth delays against log surviving fraction was linear below a fraction of 0.03, but failed to extrapolate to the origin. Following a single dose of 15 Gy to the tumour, DNA-precursor incorporation, labelling and mitotic indices were depressed for 7 days. Tumour cellularity, measured as DNA/g tumour, was reduced and the rate of increase of total clonogenic cells slower than after complete tumour recovery. From Day 7 to Day 9 all indices of proliferation recovered to about control levels, clonogenic cell numbers increased more rapidly and tumour cellularity was restored. Repopulation of the tumour therefore appeared to take place mainly after Day 7. Incorporation of [3H]-TdR into tumour DNA reached twice the control values on Day 9. The rate of tumour growth accelerated after the initial decrease, and maximum tumour growth rate was also twice the control values on Day 13. Accelerated growth rates in irradiated tumours, above those of control tumours, occurred 10-16 days after treatment. The effectiveness of sequential therapy may therefore be improved if given during this period of of accelerated tumour growth.