Tumour tissue oxygenation has been measured in man during carbogen breathing (95% O2, 5% CO2) using a commercially available polarographic electrode system (Eppendorf p02 histograph). At least 200 tumour measurements in each of 17 patients with accessible tumours were taken before, and subsequently continuously after the commencement of carbogen breathing for periods of 10 to 30 min. In 12 out of 17 patients studied there was a significant increase in median tumour p02 during the first 10 min of carbogen breathing (range 9 to 1800%). There was an initial rapid increase in tumour p02 which was maintained until 8 to 12 min, but then decreased throughout the subsequent treatment period. Although there was a reduction in the proportion of point measurements < or = 10 mmHg in 11 out of 13 patients, during carbogen breathing, measured points of < or = 2.5 mmHg were only eliminated in three out of 11 tumours. The time course has implications for the planning of clinical trials utilising radiotherapy with carbogen breathing.