The aim of this work was to study influence of overall treatment time on response of an experimental tumour to fractionated radiotherapy. Rhabdomyosarcoma R1H of the rat was treated by fractionated irradiation applying 30 fractions in different overall treatment times of 10, 18.5, 29, 39, and 67 days. The doses per fraction ranged from 1.50 to 2.67 Gy. Tumour response was assessed by net growth delay and by an in vitro colony assay. The results indicate that repopulation of the tumour with clonogenic tumour cells slowed down during treatment leading to an average doubling time of clonogenic tumour cells of 8 +/- 1 days, which is considerably longer than that of unirradiated tumours (3 days). The repopulation rate seemed to increase after about 3 weeks of treatment with a doubling time of tumour clonogens of 5 +/- 1 days, which was still lower than in control tumours.