Direct measurements using 200 micron oxygen cathode electordes have been made in tumour and subcutaneous tissues of anaesthetized C3H mice treated with misonidazole. In air-breathing animals a six-fold increase in tumour oxygen cathode readings was observed by 23 min after i.p. injection of misonidazole at 0.5 mg/g body weight. A similar rise, but of 2.5 times, was observed for the subcutaneous tissues. Comparison of tumour oxygen readings for drug-treated and control mice breathing 100% O2 at different pressures showed markedly increased levels for the drug-treated animals. Tumour response to radiation was measured for 250 kV X-rays and 15 MeV electrons. In each case, when a dose of radiation was combined with pre-treatement with misonidazole an increased radiation response was observed when compared with the same dose of radiation given alone. At the highest dose (40 Gy) there was a suggestion of an increased normal tissue reaction. The significance of the measurements is discussed.