The response of the adductor of the thumb to ulnar nerve stimulation (0.1 Hz) was evaluated during continuous infusion of succinylcholine (SCh) in 20 infants anesthetized with halothane (1%) and N2O/O2. Train-of-four stimulation (2 Hz for 2 s) was used to differentiate between phase I and phase II block. Some infants were very resistant to the neuromuscular effects of SCh. These infants (Group 1) were younger in age, 57 ± 15 days (mean ± SE) and required 24.6 ± 3.3 mg · kg-1h-1SCh to achieve more than 90% depression of the twitch. Older infants (Group 2), 188 ± 33 days, required significantly less (P< 0.001) SCh (8.7 ± 0.5 mg · kg-1h-1) to achieve the same degree of twitch suppression. Group 1 infants recovered from the effects of SCh very rapidly. After 10 mg/kg SCh, the train-of-four ratio in Group 1 infants recovered to 75% in 4.7 ± 0.6 min, whereas it took 34 ± 10 min in Group 2 infants (P< 0.01). Tachyphylaxis developed in infants after 3.6 ± 0.3 mg/kg (mean ± SE) and phase II block after 5.3 ± 0.7 mg/kg (P< 0.05) SCh. Combining the data of infants with that of children from a previous study conducted in a similar fashion1resulted in significant correlation (P< 0.001) between the log age and SCh requirement. The rate of administration of a continuous infusion of SCh in infants should be based upon the response of infants and not on a fixed dose (mg · kg-1· h-1).