Split dose irradiation with 2 X 2.5 Gy at intervals of 3 to 48 h was compared with the effect of a single irradiation with a 5 Gy dose in Bp8 mouse ascites sarcoma growing in vivo. The total number of cells and, using rapid-flow cytofluorometry, the proportions of cells in the various parts of the cell cycle were determined up to 10 days after irradiation. In addition, from sequential analyses of the total number of cells in the various compartments of the cell cycle, the flow of cells through the cell cycle was calculated. As judged by the growth in the total number of cells, a rapid repair of at least 20 per cent per h up to 3 h is followed by a slow repair of about one per cent per h during the following 45 h. The proportion of cells in the cell cycle following the conditioning dose did not differ enough to expect a significant influence on the mean radiation sensitivity of the cell population. At split time intervals of 12 to 48 h, after a transient early increase of the flow of cells through the cell cycle, the flow of cells was markedly reduced in all parts of the cell cycle with a maximum 24 to 48 h after the second dose of irradiation. This increase in the cell cycle time was not found following single dose irradiation and may influence the repair of potentially lethal cell damage.