Studies were made to ascertain the relationship between arterial and end-tidal carbon dioxide tension in 19 patients during caesarean section under general anesthesia(Group 2). 27 nonpregnant patients scheduled for abdominal hysterectomy were also performed as a control group(Group I). 15 and 30 minutes after the induction of anesthesia in group I, and just before uterine incision, 15 minutes following induction in group 2, arterial blood was sampled for PaCO analysis and PETCO2(end-tidal CO2 tension) determined at the same time. In group 1, there was a statistically significant differences between PaCO2 and PETCO2 but in group 2, no differences between them were shown. The highly significant correlations between PaCO2 and PETCO2 in both groups were found. On the basis of the above results, we recommend that non-invasive measurement of PETCO2 can be used reliably in stead of measuring PaO2 to evaluate the ventilatory status during anesthesia in patients for caesarean section.