Simple O2 mask has been used in patient under regional anesthesia for increasing the FiO2, especially in the aged and the pregnant. The relationships between maternal FiO2 and umbilical arterial (UA) and venous (UV) PO2, PCO2, pH, and neonatal Apgar score were studied in 45 patients receiving Cesarean section under epidural anesthesia. 2% xylocaine 18-20 ml with adrenaline 1:200,000 was used to attain the sensory level of T-4. Patients were allocated randomly into three groups. Group I, acting as a control group, breath only room air during the course of anesthesia. Group II was breathing through a simple face mask with an oxygen inflow of 6L/min. Group III was breathing oxygen with a flow rate of 10L/min through a simple face mask. UA and UV blood samples were collected separately at the time of delivery for blood gas analysis. The 1-min, and 5-min Apgar scores were recorded also. Mean values of the UA blood samples for the 3 groups (Gp. I, Gp. II and Gp. III respectively) were: PO2--16.50 mmHg, 20.20 mmHg and 19.50 mmHg; PCO2--49.20 mmHg, 48.10 mmHg and 50.3 mmHg; pH--7.31, 7.30 and 7.30. Mean values of the UV blood samples for the 3 groups (Gp. I, Gp. II and Gp. III respectively) were: PO2--28.6 mmHg, 36.9 mmHg and 36.5 mmHg; PCO2--38.20 mmHg, 38.80 mmHg and 40.40 mmHg; pH--7.36, 7.36 and 7.34. There was a significant increase in UA and UV PO2 when using a simple O2 mask (p < 0.001).(ABSTRACT TRUNCATED AT 250 WORDS)