Two commercially available pulse oximeters were found to read reliably early during periods of low cardiac index (CI) and low peripheral temperature (Tp). The lowest mean CI and mean Tp values at which a reading could be obtained were 2.4 L/min m: and 26.5°C. respectively. The highest mean systemic vascular resistance index was 2930 dyne -sec/cm5m2. The reliability of one monitor was equally good both in a normal and low perfusion mode (i.e. when the monitor displayed a low-quality signal message). The use of locally applied vasodilating creme widened the physiologic limits within which reliable measurements could be obtained. All oximeters and sensors overestimated slightly the saturation as compared to the hemoximeter readings. possibly because of a high mean carboxyhemoglobin concentration that was found to be present.