Fifty healthy mothers, with normal placental function, were anaesthetised with ketamine for Caesarean section. Anaesthesia was maintained with nitrous oxide, oxygen, muscle relaxants and controlled ventilation. Surgery was conducted in the lateral tilt position. Arterial blood samples were drawn from the mothers, and from the vessels of a double clamped section of umbilical cord, for blood-gas analysis. Results obtained were compared with those of a previous series anaesthetised with thiopentone, nitrous oxide, oxygen and muscle relaxants. Eight infants were clinically depressed, judged on the basis of their modified Apgar score 2 minutes after delivery. The average time to sustained respiration (TSR) was 58.1 seconds. The mean maternal pH and base excess values in the ketamine group were significantly greather than those reported after thiopentone anaesthesia. Mean Uv and Ua pH levels were also significantly higher after ketamine; in contrast, the average fetal base excess values did not differ from those obtained previously with thiopentone. The mean (Ma-Uv) and (Ma-Ua), pH gradients were 0.019 and 0.025 pH units greater respectively in the ketamine group compared to the thiopentone (P less than 0.005). The average (Uv-Ua) PO2 gradient was 3.4 mmHg less after ketamine anaesthesia (P less than 0.005). A significant inverse correlation was observed relating the I-D interval to the Ma and Ua pH values. Maternal arterial base deficit values appeared to increase with delay in delivering the fetus. Prolongation of the uterine incision to delivery (U-D) interval was associated with a decrease in Ua pH and base excess values. (Ma-Ua) pH and base excess gradients increased with lengthening of the U-D interval. No convincing evidence of awareness during anaesthesia was found during the study. Five patients, appeared to be hallucinated in the immediate post-anaesthetic period. Unpleasant dreams were reported in 5 instances. In this study ketamine appeared to be unassociated with significant biochemical asphyxia, but may have been responsible for some element of drug induced neonatal depression. In view of our own experience and that of other workers, it is suggested that ketamine induction for Caesarean section should be re-evaluated using a lower dose of the drug.