Arterial PCO 2 (PaCO 2) can be continuously and noninvasively estimated by monitoring peak expired CO 2 tension (PpeCO 2). The practice of calibrating the estimate by an initial measurement of PaCO 2 assumes that the difference in PCO 2 tension between arterial blood and expired gas P (a-pe) co 2 remains constant. We examined the stability of P (a-pe) CO 2 during anesthesia in 15 patients undergoing major surgery. Mean P (a-pe) CO 2 values ranged from 0.8–7.9 torr with maximum P (a-pe) CO 2 values ranging from 4.5–13.0 ton. Calibration of P (a-pe) CO 2 based on a single initial measurement of PaCO 2 often over-or underestimated PaCO 2. Mean estimated PaCO 2 from calibrated P (a-pe) CO 2 varied from-7.9–6.4 torr with extreme estimates of—12.8–12.3 torr. No consistent correlation was shown between P (a-pe) CO 2 and duration of anesthesia, variations in ventilation, blood pressure, blood-gas tensions, PpeCO 2 or temperature. We conclude that estimation of PaCO 2 by monitoring PpeCO 2 is not invariably reliable.