Studies were performed on young (5-, 10-, 15-, 30- and 60 days old) and senescent (540-days old) female Wistar rats. The rats were administered (i.p.) phenobarbital (150 mg per 1 kg body weight) twice: 72 and 48 h before being killed. In smooth (SER) and rough (RER) hepatic microsomes the activity of mixed functions oxidases was studied. In particular age groups phenobarbital has a changeable effect on the mixed functions oxidases. All the examined components except for the b5 cytochrome content and the NADH-cytochrome b5 reductase activity are induced. In the group of young rats the highest induction was observed in 15- and 30 days old animals and the lowest--in 5- and 60 days old ones. Only for cytochrome P-450 content and NADPH-cytochrome c activity in rough microsomes and protein content in smooth microsomes was the maximal induction highest in 60 days old rats. Small decrease in the protein content in smooth microsomes in 540 days old animals make one suggest the senescent rats are less susceptible to phenobarbital.