This study was undertaken to see if infants are more sensitive than children to a combination of atracurium and vecuronium in an equipotent dose ratio: (microgram: microgram) 5:1 in infants and 4:1 in children. We studied 15 infants (1-11 months old) and 15 children (3-10 yr old) during nitrous oxide-oxygen-alfentanil anaesthesia. Neuromuscular function was recorded by adductor pollicis EMG. An individual dose-response curve of the atracurium-vecuronium combination was determined for every patient and its potency compared with that of the parent agents alone. The combination was significantly more potent than one parent agent, both in infants (P < 0.01) and in children (P < 0.0001). However, infants were less sensitive than children to synergism produced by the atracurium-vecuronium combination: if the ED50 dose of the parent agent is defined as one dose equivalent, then the mean ED50 doses of the combination were 0.81 (SEM 0.05) and 0.64 (0.03) dose equivalents in infants and children, respectively (P < 0.01). We suggest that an interaction between two binding sites of competitive neuromuscular blocking agents in postsynaptic acetylcholine receptors may explain both the synergism and sensitivity of infants to non-depolarizing neuromuscular blocking agents.