Abstract Objective: The cytochrome P450 (CYP450) superfamily is implicated in important life processes, including metabolism of many molecules. CYP3A account for the largest portion of CYP450 proteins in human, including CYP3A4, CYP3A5 and CYP3A7. The purpose of this study was to investigate the immunohistochemical expression of CYP3A4 and CYP3A7 in human liver at different post-conceptional (PC) ages. Methods: Human liver samples from 30 fetuses and newborns were, clustered according with the PC age, routinely processed for immunohistochemical analysis of CYP3A4 and CYP3A7. Results: CYP3A4 was positive in all but two cases, CYP3A7 was positive in all but one case, which was negative also for CYP3A4. Conclusions: Our data on immunohistochemical detection of CYP3A4 and CYP3A7 during development show that CYP3A4 expression is not restricted to the post-natal age, being the immunostaining for both CYP3A4 and CYP3A7 identical after 25 weeks of PC age, thus the relationship between these CYP450 isoforms should be considered much more complex than previous thought. A high interindividual variability was observed among subjects at all gestational age. The variable CYP3A expression suggests the existence of a marked interindividual variability in drug metabolism during the intrauterine life and in perinatal period.