Transcutaneous carbon dioxide tensions (tcPCO2) measured with a heated electrode were compared with arterial carbon tensions (PaCO2) in 28 patients. Seventy-eight observations were made. At a skin electrode temperature of 44 degrees C the tcPCO2 was significantly higher than PaCO2 but changes in PCO2 detected by both measurements were closely correlated. The correlation coefficient between tcPCO2 and PaCO2 was 0.92 (P less than 0.001). The 90% response time of the electrode in vitro was less than 1 min, and in vivo stabilization of the recording occurred in less than 15 min. It was found that the transcutaneous PCO2 electrode can be employed usefully in intensive care monitoring of adult patients especially when weaning them from artificial ventilation.