Alfentanil pharmacokinetics and protein binding were determined in 20 children aged 10 months-6.5 yr. The data were compared with those from 10 adult patients. Eighteen children received a single i.v. dose of alfentanil 20 micrograms kg-1. The apparent volume of distribution (V beta) did not differ between the two groups. The degree of plasma protein binding was also similar in children and adults with mean free fractions of 11.5 +/- 0.9% (+/- SD) and 11.8 +/- 3.9%, respectively. There were marked differences in the elimination half-life of alfentanil (63 +/- 24 min in children; 95 +/- 20 min in adults (P less than 0.001] and plasma clearance of alfentanil (11.1 +/- 3.9 ml min-1 kg-1 in children and 5.9 +/- 1.6 ml min-1 kg-1 in adults (P less than 0.001].