We measured free and total venous bupivacaine plasma concentrations in fourteen infants and children aged 6 days (2800 g) to 9 years (27 kg) undergoing epidural anaesthesia. An initial bolus of 0.5 ml.kg-1 bupivacaine 0.25% was followed by a continuous infusion administered one h after bolus over a period of seven h (first hour 0.25 ml.kg-1.h-1 0.25%; then reduced to 0.125%). Although total bupivacaine plasma concentrations were within acceptable limits (< 1.5 micrograms.ml-1), four of the seven infants showed adverse reactions. Maximum plasma concentrations of free bupivacaine were significantly higher in infants (P < 0.05) than in older children. We conclude that toxicity may be underestimated when only measuring total bupivacaine concentrations. In young infants the bupivacaine dose administered for continuous epidural anaesthesia should be further lowered below recommended concentrations and the patients closely observed for possible adverse reactions.