The myotonic effects and duration of action of several doses of suxamethonium were determined in 24 infants and 16 children during thiopentone-fentanyl-nitrous oxide anaesthesia. Infants received suxamethonium 2, 3 or 4 mg kg-1; children received 1 or 2 mg kg-1. The increase in muscle tone during onset of neuromuscular block was independent of dose. Onset of block was faster in children who received suxamethonium 2 mg kg-1 compared with those who received 1 mg kg-1, and in infants given 2 mg kg-1 than in children given the same dose. Compared with adults given suxamethonium 1 mg kg-1, infants required 3-4 mg kg-1 and children at least 2 mg kg-1 to produce 6-8 min of neuromuscular block. These results provide a clear indication for increasing the intubating doses of suxamethonium in infants and children, and a explanation for the unduly high rate of "masseter spasm" in some paediatric centres.