Total and free plasma concentrations of carbamazepine (CBZ) and carbamazepine-10,11-epoxide (CBZ-E) were determined in 39 children (aged 3 to 10 years) and 79 adults (aged 15 to 65 years) receiving long term treatment with CBZ alone or in combination with phenobarbitone (PB).Compared with the corresponding age groups treated with CBZ alone, adults and children receiving PB co-medication showed lower total and free CBZ concentrations, similar CBZ-E concentrations and higher CBZ-E/CBZ ratios. Among patients on CBZ alone, children had at any given dose lower total and free CBZ and CBZ-E concentrations than adults. Lower CBZ levels in children than in adults were also found among patients receiving phenobarbitone in combination. CBZ-E/CBZ ratios did not differ significantly between children and adults. These data provide evidence that children show an elevated free CBZ clearance with a metabolic pattern different from that observed during phenobarbitone induction.