In rats, the activity of some hepatic redox-enzymes was measured following the administration of the enzyme-inducing substances phenobarbital, α-hexachlorocyclohexane (α-HCH = α-benzene hexachloride), CFT 1201 (phenyl-diallylacetic acid ester of diethylaminoethanol), or 3,4-benzpyrene. In particular, the time course of changes in enzyme activities was studied. 1. α-HCH and CFT 1201, like phenobarbital, increase the concentration of cytochrome P450 and accelerate the NADPH-dependent reduction of cytochrome c. All three substances reduce the activity of NADH-cytochrome c-reductase. On the other hand, only α-HCH, increases aldehyde oxidase activity in the hyaloplasm. 2. Phenobarbital and α-HCH both enhance the rate of demethylation of aminopyrine considerably more than they increase the concentration of cytochrome P450. Similarly, benzpyrene produces an increase in the rate of hydroxylation of acetanilide that is higher than the increase in concentration of P450. 3. CFT 1201 inhibits acetanilide-oxidation in vivo, but not in vitro. 4. Phenobarbital induces microsomal enzymes more strongly than does α-HCH, whereas this latter compound-as earlier observations have shown-stimulates liver cell proliferation much more than phenobarbital. © 1969 Springer-Verlag.