1. The content and specific activities of inducible cytochrome P-450 enzymes were determined in liver microsomes of rats of various ages after maximal induction with phenobarbital, isosafrole of 3-methylcholanthrene, and in untreated animals. 2. With age an increase in liver weight was observed both in untreated rats and in maximally induced ones; the microsomal protein content/g of liver decreased with age in untreated animals but not in induced ones. Total cytochrome P-450 content/mg microsomal protein remained unchanged with age in all experimental groups. 3. Immunologically detectable levels of cytochrome P4501A1/1A2 and 2B1/2B2 remain unchanged with age both in untreated animals and in maximally induced ones. 4. Several cytochrome P-450 activities showed an age-related decrease in untreated animals, but no change with age was observed in the activities of cytochrome P4501A1, 2A2 and 2B1/2B2 in rat liver microsomes. This indicates that ageing affects only the activity of some constitutive forms of cytochrome P-450 in male rats, but not the activity of inducible types of P-450. 5. Although previous results indicated decreased inducibility of the cytochrome P-450 mRNA levels with age, the present study clearly demonstrates that this is not reflected in decreased enzyme levels or activities after maximal induction. From this it is concluded that the decreased mRNA levels might rather be reflected in a decreased rate at which maximal induction can be achieved.