Activity of enzymes participating iH metabolism of xenobiotics was studied after their induction by means of phenobarbital in rats of Wistar strain within first weeks of life. After administration of phenobarbital into the neonatal rats content of cytochrome P-450 and activity of NADPH-cytochrome c reductase were increased in liver microsomes. The rates of N-demethylation of benzphetamine and benz(alpha)pyrene hydroxylation were also increased after administration of the inducing agent. Immunochemical studies with antibodies towards some forms of cytochrome P-450 showed that enzymatic forms of cytochrome P-450 PB were induced; this enzymatic form is predominant in adult animals treated with phenobarbital. When the metabolism of benzphetamine and benz(alpha)pyrene was inhibited by antibodies, several forms of cytochrome P-450, involved in these metabolic reactions were found in neonatal rats, whereas in the adult animals only one form of the enzyme exhibited activity towards these substrates.