The optimal dose of remifentanil needed to produce successful intubating conditions following inhalation induction of anaesthesia using 5% sevoflurane without the use of neuromuscular blocking drugs, was investigated in 25 children aged 3-10 years. Sixty seconds after inhalation induction of anaesthesia using sevoflurane 5% in 100% oxygen, a predetermined dose of remifentanil was injected over 30 s. The dose of remifentanil was determined using the modified Dixon's up-and-down method (0.2 microg x kg(-1) as a step size). The first patient was tested at 1.0 microg x kg(-1) remifentanil. Ninety seconds following the bolus administration of remifentanil, the child's trachea was intubated. The optimal bolus dose of remifentanil required for successful tracheal intubation was 0.56 (0.15) microg x kg(-1) in 50% of children during inhalation induction using 5% sevoflurane in the absence of neuromuscular blocking drugs. Using probit analysis, the 95% effective dose (ED(95)) of remifentanil was 0.75 microg x kg(-1) (95% confidence limits 0.63-1.38 microg x kg(-1)).