Catheter-tip sensors used to measure arterial and tissue PCO2 produced a baseline drift less than 2 torr during 1 to 5 h of monitoring. Clot formation during PaCO2 monitoring was avoided by continuous infusion of a small amount of heparin solution around the sensor. PaCO2 sometimes fluctuated periodically during spontaneous breathing but not during mechanical ventilation. Subcutaneous tissue PCO2, monitored by placing the sensor under the skin on the chest, showed an almost parallel movement to PaCO2 with a time delay of a few minutes. This PCO2 sensor was durable and accurate for up to 5 h continuous clinical monitoring.