The pharmacokinetics of alfentanil were studied during the first four days after birth in 22 ventilated preterm infants who were all receiving muscle relaxants. Five minutes after a single dose of 20 micrograms/kg alfentanil median serum concentration was 66 ng/ml (range: 20-606). The median clearance was 0.87 ml/kg/min (range: 0.4-9.62) and median elimination half life 321 mins (64-1251). There were wide differences in the manner in which individual infants handled the drug and transient depression of blood pressure and heart rate was observed. These data were used to calculate an infusion dosage. In four infants 20 micrograms/kg alfentanil given by infusion over 30 minutes followed by 5 micrograms/kg/hour produced steady state median alfentanil concentrations of 54.5 ng/ml (range: 7-73 ng/ml) with no evidence of drug accumulation.