Apparatus suitable for continuous, ambulatory transcutaneous PCO2 monitoring is described. This consists of a Stow-Severinghaus transcutaneous sensor with a miniaturized amplifier and recorder. Its performance characteristics and vulnerability to artefact have been assessed. The procedure for prolonged ambulatory recording is outlined. The response time (for 67% of change) was 13 s in dry gas and 5 min on skin (the reasons for this difference are discussed). With 28 subjects, the between subject correlation (r) of transcutaneous and arterial PCO2 measurements was 0.97. After equilibration, recorded values drifted only occasionally. When it did occur, the average was 0.03 kPa X h-1 in dry gas and 0.1 kPa X h-1 on skin. There was no evidence of artefact due to changes in temperature or blood flow. Movement artefact were eliminated using an adequate attachment procedure. A number of healthy volunteers were monitored for up to 12 h while freely ambulant. In all subjects, small fluctuations in transcutaneous PCO2 were observed, of less than 0.5 kPa. The apparatus is of value in the assessment of conditions and situations in which acute change in ventilation has been implicated and for investigation of normal respiratory physiology with freely ambulant subjects.