In 15 patients anaesthesia for elective Caesarean section was maintained with 50% nitrous oxide in oxygen and a 0.65% halothane supplement. In a further 15 mothers a 0.2% halothane supplement was used. In those mothers receiving 0.65% halothane intermittent measurements were made of maternal arterial halothane concentrations during the uptake and the excretion of the agent. At delivery the foetal umbilical venous concentrations were measured also. The mean time between administering halothane and delivery was 10.5 min (SD 3.5). The mean maternal arterial halothane concentration at delivery was 6.03 mg/100 ml (SD 0.75) and the mean umbilical vein concentration was 2.13 mg/100 ml (SD 0.69). The regression of Apgar scores at 1 min after delivery on umbilical vein halothane concentration at delivery was not significant. In the mothers receiving 0.2% halothane measurements of halothane concentration were made in five patients only. The mean maternal arterial halothane concentration at delivery was 1.56 mg/100 ml (SD 0.52) and the mean umbilical vein and artery concentrations were 0.8 and 0.38 mg/100 ml respectively. The use of 0.2% and 0.65% halothane supplements prevented awareness in all the mothers. However, dreaming occurred in two patients given a 0.2% halothane supplement. Studies are required to establish the minimum halothane supplement required to prevent awareness in a larger series of patients.