The dose-response relationship and time course of action of neostigmine were determined in infants and children during N2O-halothane anesthesia. d-Tubocuranine (dTc) was administered by continuous i.v. infusion to maintain twitch tension of the adductor pollicis at constant 90% depression. Then 12 infants (3-48 wk) and 15 children (1-8 yr) were given neostigmine (6.25, 12.5 or 25 .mu.g/kg) with atropine while the dTC infusion was continued. The ED50 dose of neostigmine (which produces 50% antagonism of dTc-induced neuromuscular depression) was 13.1 .mu.g/kg in infants and 15.5 .mu.g/kg in children. This can be compared with a value of 22.9 .mu.g/kg in adults studied under similar anesthetic conditions. The time to 30, 50 and 70% of peak antagonism was similar for infants, children and adults. The course of antagonism was followed past its peak effect in 15 subjects. In these subjects, duration of antagonism was similar to values in adults. The pharmacokinetics of neostigmine were determined. Three groups of 5 patients (infants, 2-10 mo.; children, 1-6 yr; and adults, 29-48 yr) were given neostigmine as a 2 min i.v. infusion. Infants received 100 .mu.g/kg; children and adults, 70 .mu.g/kg. Blood samples were obtained intermittently for 4 h, and concentrations of neostigmine were determined using a high-pressure liquid chromatographic technique. These data were fitted to a 3-compartment pharmacokinetic model. Distribution half-lives and distribution volumes were similar for infants, children and adults. Elimination half-life was shorter for infants (39 .+-. 5 min, mean .+-. SD) and children (48 .+-. 16 min) compared with adults (67 .+-. 8 min). The time course of onset and duration of antagonism is evidently similar for infants, children and adults. The dose of neostigmine required to antagonize dTc-induced neuromuscular blockade is lower in infants and children than in adults. This difference in dose requirement cannot be explained by age-related changes in volume of distribution. These findings are in contrast to the commonly held (but not documented) belief that infants and children require larger doses of neostigmine.