2/3 Hepatectomy (HX), but also sham operation (SO) decreased liver dry mass for the first 3-4 days after operation both in 10- and 60-day-old male Wistar rats, whereas body mass development was not altered. Both SO and HX decreased ethylmorphine N-demethylation (EN) and ethoxycoumarin O-deethylation (EO), but distinctly more pronounced after HX. In 10-day-old rats EN and EO were in the normal range 14 days after SO and HX. After 3 weeks EN even exceeded the control level. In 60-day-old rats SO alone resulted in decreased EN and EO even 3 weeks after operation. Compared with SO-controls EN was lower but EO was in the same range in HX rats 14 days after operation. Neither in 10- nor in 60-day-old rats inducibility by phenobarbital (PB) could be demonstrated 24 h after HX with cytochrome P-450 concentration (P-450), EN and EO as parameters. But inducibility by betanaphthoflavone (BNF) was significant for P-450, EN and EO in 10-day-old rats and also for P-450 and EO in 60-day-old rats. In the young adult rats EN was depressed by BNF. In rats 10 days old at HX inducibility by PB was highest 3-7 days after HX, but inducibility by BNF after 14 days. In 60-day-old rats inducibility by PB was highest for EO 1 day after HX. These results indicate that ontogenetic development can be enhanced by induction, that during development regeneration can be superimposed and both induction and regeneration can superimpose ontogenetic development. During these processes different monooxygenases are under differential control.