We have studied the feasibility of improving tumour oxygenation with hyperthermia at modest temperatures which are achievable with the use of presently available clinical hyperthermia machines. FSaII tumours grown s.c. in the leg of C3H mice and R3230 AC tumours grown s.c. in the leg of Fischer rats were heated with a water bath and the tumour pO2 was determined with an Eppendorf pO2 histograph. The median pO2 in 7-8 mm diameter control FSaII tumours was 6.5 +/- 0.5 mmHg and it increased to 16.6 +/- 1.1 mmHg when the tumours were heated at 41.5 degrees C for 1 h. The median pO2 in 10 mm diameter control R3230 AC tumours was 3.7 +/- 0.3 mmHg. Heating at 42.5 degrees C for 30 min increased the median pO2 in the R3230 AC tumours to 12.2 +/- 1.8 mmHg. The pO2 in FSaII tumours measured 24 h after heating at 41.5 degrees C for 1 h was still higher than the pO2 before heating. The % frequency of pO2 values lower than 5 mmHg decreased markedly when the tumours were heated at the modest temperatures mentioned above. Modest temperature hyperthermia (MTH) may be an efficient and useful means to improve the oxygenation of human tumours.