The 13C-AP breath test is shown to be a convenient, noninvasive method to monitor velocity and capacity of P450-dependent AP N-demethylation in infancy and childhood. According to 13C-AP breath tests, neonates have a very low capacity to eliminate 13CO2, which is only 15 to 21% of the activity in adults. During the first year of life AP N-demethylation increases to reach its maximum at about 2 years; afterwards a slight decrease occurs. In 25 neonates exposed prenatally to different antiepileptic drugs 13C-AP breath test was efficiently used to prove that cytochrome AP N-demethylation was considerably stimulated. After primidone/phenobarbitone, especially in combination with phenytoin, 13C elimination reaches and even surpasses the range for older children. Valproate exposure during fetal life is not consistently followed by a significant increase in AP N-demethylation. The enzyme induction demonstrated by 13C-AP breath test was often accompanied by accelerated metabolic clearance and shortened half-life times of transplacentally acquired antiepileptic drugs. There was good agreement between 13C-AP breath tests and pharmacokinetic data for primidone/phenobarbitone but not for phenytoin. In contrast, in the case of phenytoin exposure during pregnancy the pharmacokinetic parameters and the 13C breath test data will transport very different informations about enzyme induction in these neonates.