Neonatally administered phenobarbital has been shown to cause a permanent, but delayed overexpression of hepatic drug-metabolizing enzymes, occurring at the time of sexual maturity. The present studies indicate that these above-normal levels of hepatic monooxygenases are not a result of a persistent overexpression of CYP2B1 and 2B2 proteins, the major phenobarbital-inducible isoforms of cytochrome P450. However, early exposure to the barbiturate permanently alters (i.e., imprints) the inductive responsiveness of CYP2B1 and 2B2 to subsequent phenobarbital challenge in adulthood. That is, neonatal administration of therapeutic-like levels of phenobarbital causes an overinduction (approximately 30-40%) of CYP2B1 and 2B2 mRNAs, proteins and specific catalytic activity (androstenedione 16 beta-hydroxylase) levels when the rats are rechallenged as adults with as little as 1 mg or 10 mg/kg b.wt. of the barbiturate. This "latent" defect in the inductive mechanism is associated in both sexes with an abnormality in the circulating growth hormone profiles characterized by subnormal secretory pulses. Because endogenous growth hormone normally inhibits phenobarbital induction of CYP2B1 and 2B2, and the level of inhibition is directly related to the height of the secretory pulse, we have proposed that the overinduction of CYP2B1 and 2B2 in adult rats neonatally exposed to phenobarbital results, at least in part, from a "deinhibition" by the subnormal pulse amplitudes in the plasma growth hormone profiles.