The activity of the mixed function oxidase (MFO) system of the lung and liver of goats were measured in three age groups. The induction of the MFO system by sodium phenobarbital (PB) and the inhibition of piperonyl butoxide (BT) in each age group were also determined. 2. The cytochrome P-450 contents of the liver and lung were less in the newborn than that of the weanling or mature goats. There was no significant change in these parameters between weanling and mature goats. 3. The activity MFO system in the lung was consistently lower than that of the liver in all age groups. 4. Administration of PB induced hepatic cytochrome P-450; however, PB was not a suitable inducer in the lung of goats. BT inhibited this enzyme system in weanling and mature but not in newborn goats.
