The effect on accelerated repopulation of fractionated radiotherapy with changing weekly dose was studied in mouse oral mucosa. The basic conventional treatment protocol comprised five daily fractions per week, 3 Gy per fraction, given over 3 weeks. In dynamic treatment schedules the same total dose was delivered by giving 5 x 2, 5 x 3 and 5 x 4 Gy, or the reverse sequence. In concomitant boost experiments five additional daily doses of 1 or 2 Gy respectively were applied either during the first or last week of the conventional treatment. Graded terminating test doses were given in order to establish dose-effect curves for complete mucosal denudation. Higher initial doses consistently resulted in longer latent times between top-up irradiation and complete epithelial denudation, corresponding to significantly higher cell densities at the time of test irradiation. The top-up ED50's served as a measure of repopulation efficacy. The difference in repopulated dose between schedules with initial or terminal dose concentration was 10% for dynamic fractionation (p = 0.02) and about 7% in the 2-Gy boost protocol (p = 0.13), and no difference was observed with boost doses of 5 x 1 Gy. In conclusion, a clear though moderate increase in both cell density and residual tissue tolerance can be achieved by protocols with substantially higher doses in the first treatment week.