Transcutaneous carbon dioxide and oxygen tensions (tc-pCO2 and tc-pO2) were measured in seven healthy adult volunteers during hyperventilation in atmospheric air and during CO2 inhalation. Three skin sensors were applied to each subject: an O2 electrode, a CO2 electrode, and a combined O2-CO2 electrode, each heated to 44 degrees C. We observed close correlation between tc-pCO2 and capillary-pCO2, the relation being close to that calculated from the anaerobic temperature coefficient of pCO2 in blood. For O2, on the other hand, the relationship between transcutaneous and capillary values appeared more complex. Electrode drift during in-vivo monitoring was greater for pCO2 (up to 12%) than for pO2 (up to 7%), but generally we observed no differences in drift between the combined and the single electrodes. We conclude that tc-pCO2 measured with a single or a combined electrode reliably predicts capillary-pCO2 in healthy adults and that changes are rapidly observed. Our conclusions regarding tc-pO2 values are less definite because of uncertain interpretation of the capillary-pO2 values.