The effect of fractionated irradiation on the oxygenation status of experimental tumours was investigated using polarographic assessment of the pO2 distribution. Since an improvement in tumour oxygenation could simply be the result of tumour shrinkage, a comparison of pO2 readings of untreated size-matched control tumours was performed. Irradiation was carried out using 6 fractions of 6 Gy applied within 11 days. A comparison of polarographic pO2 data with size-matched untreated tumours revealed a significant improvement in oxygenation after the irradiation. The median pO2 was 0.9+/-0.1 mm Hg for unirradiated tumours at a volume of 180 mm3, while the corresponding data for irradiated tumours of comparable size were 2.3+/-0.5 mm Hg on day 21 and 4.8+/-0.9 mm Hg on day 28 after start of irradiation. From these results it can be concluded that the improvement of oxygenation after fractionated irradiation is not solely the result of a reduced tumour volume.