The CYP2D6 protein is a polymorphic isoenzyme involved in the biotransformation of several drugs including the probe drug dextromethorphan. The rise in the protein concentration, immunochemically determined with a specific antibody, was shown to occur within the first week following birth, whatever the gestational age at birth. In fetuses, the concentration of hepatic CYP2D6 protein was very low or undetectable in 70% of samples tested. In the remaining 30%, its concentration was comparable to that of newborns aged 1-7 days. This early rise was associated with spontaneous abortion in 70% of positive samples, whereas in fetuses with an intermediate CYP2D6 protein concentration, 80% were from induced abortions. The rise in CYP2D6 protein was associated with the developmental onset of dextromethorphan O-demethylation, but not N-demethylation, even if activity was lower in fetal than in neonatal and in adult liver microsomes. Lastly, the CYP2D6 RNA is detectable earlier than the protein and exhibits a peak of hepatic accumulation in newborns, before declining in adulthood. A positive correlation between RNA accumulation and protein concentration can be demonstrated only in the adult. This suggest that regulation is primarily at the transcriptional level, but cannot rule out the participation of post-transcriptional events in the regulation process throughout ontogenesis.