Neonatal exposure to certain xenobiotics has been shown to alter hepatic metabolism in adult rats in a manner that indicates long‐term changes in enzyme regulation. Previously, we have observed changes in adult testosterone metabolism and in cytochrome P‐450 (P‐450) mRNA levels in animals neonatally exposed to phenobarbital (PR) or diethylstilbestrol (DES). In order to test for other enzyme alterations, we used Western blot procedures for specific P‐450s to analyze hepatic microsomes from adult rats (24 wk old) that had been exposed neonatally to DES, PB, 7,12‐dimethylbenz[a]anthracene (DMBA), or pregnenolone 16α‐carbonitrile (PCN). The most striking effects were observed in the DES‐treated males: P‐4502C6 and an immunologically similar protein were increased 60 and 90%, respectively, relative to control values, but P‐4503A2 was decreased by 44%. No changes were observed in the DES‐treated males in levels of P‐4502E1, P‐4502B, or the male‐specific P‐4502C13. Adult males neonatally treated with PB had 150% increase in levels of anti‐P4502B‐reactive protein without significant changes in the other enzymes. The DES‐ and DMBA‐treated females had increased levels of the female‐specific P‐4502C12 of 38 and 48%, respectively, but no other observed alterations. The results confirm that neonatal exposure to DES or PB can cause alterations in adult hepatic cytochrome P‐450 levels but show that these chemicals act on different enzymes. Neonatal DMBA resulted in changes in adult females similar to those produced by the synthetic estrogen DES, but did so at about two‐thirds lower dose.