The elimination kinetics of thiopentone was studied in 7 newborns delivered by Caesarean section and in their mothers who had received the drug for induction of anaesthesia. At delivery, 4-9 min after induction, drug concentrations in cord blood were half those in material blood. The mean half-life of thiopentone in the newborns was about double that in their mothers (15 vs 7 h) confirming a disposition similar to other barbiturates. For the first time renal clearance of thiopentone was estimated in the newborn; 0.074 ml/h/kg. Only 0.0007% (about 2 micrograms) of the maternal dose was recovered in the urine of newborns over 36 h. Pentobarbitone, an active metabolite, was not detected in any specimen. The findings demonstrate the reliability of current anaesthesiological technique (thiopentone-succinylcholine-oxygen) with minimal fetal exposure to the drug.