In 50 cases of Caesarean section for various indication, methoxyflurane was administered to two groups of patients in two different dosages by a Pentec-vaporizer. Blood was sampled simultaneously in the radial artery of the mother and in the umbilical vein. The methoxyflurane concentrations of both samples of blood were measured by gas chromatography. With an inspiratory concentration of 0.2 vol.-% methoxyflurane, the mean concentration was 166 mumol/1(2.75 mg/100 ml) in the maternal blood and 69 mumol/1 (1.14 mg/100 ml) in the umbilical vein. With 0.5 vol.-%, the corresponding values were 345 mumol/1 (5.72 mg/100 ml) and 137 mumol/1 (2.25 mg/100 ml) respectively. The condition of the new-born did not appear to be affected by the given doses of methoxyflurane.