Serum cholinesterase activity was measured at induction, and following anaesthesia in 41 children aged between 4 and 30 months. The median exposure to sevoflurane was 273%.min. The results did not demonstrate any significant difference in cholinesterase activity, when expressed by gram of serum proteins following inhalation anaesthesia using sevoflurane or intravenous anaesthesia using propofol. The values (SD) obtained were 175 (42) UI.g(-1) before anaesthesia and 177 (43) UI.g(-1) following anaesthesia. The only change in cholinesterase activity detected was related to heamodilution. We conclude that plasma fluoride concentration following sevoflurane administration [13.8 (4.2) microm x l(-1)] is too low to exert an inhibiting effect on in vivo cholinesterase activity and that the previously reported decrease in mivacurium requirements during sevoflurane anaesthesia is unlikely to be due to inhibition by fluoride ions.