Profound species differences and developmental stage differences as well as a lack of solid data prevent broad, sweeping generalizations in terms of statements that can be made concerning the prenatal expression of individual P450 isoforms. It is clear, however, that several of such isoforms are expressed at levels that can be toxicologically significant. At present, the greatest interest appears to be in P450s 1A1, 1B1, 2E1, and 3A7, each of which has been reported to be expressed at toxicologically significant levels or at least at potentially toxicologically significant levels during organogenesis. Reports of the expression of other P450 isoforms at later stages of gestation also have appeared in the recent literature.