Hepatic 7-alkoxycoumarin O-dealkylation activities in control and beta-naphthoflavone-pretreated mice were determined. The O-demethylation and O-deethylation activities of 7-alkoxycoumarin in control mice were almost the same values, while the O-depropylation activity was lower than those of the other reactions. The O-dealkylase activity varied markedly among the 18 strains of mice surveyed, and strain-dependent differences in the cytochrome P-450 content were also detected among the strains. beta-Naphthoflavone induced O-dealkylation activity, especially O-deethylation and O-depropylation activities, only in ddY, DS and its substrains (A2-3, A3-1, C1-2 and Nh/+). C3H/He and C57BL/6J strains, but not in DBA/2, BALB/c and KYF/2 strains. The former strains of mice are thus classified as "responsive strains" to beta-naphthoflavone and the latter as "non-responsive strains". The O-dealkylase activity in other strains of mice were not clear in responsiveness to beta-naphthoflavone. The hepatic cytochrome P-450 content in responsive strains also increased upon pretreatment of the animals with beta-haphthoflavone. The results indicate marked strain differences in basal and beta-naphthoflavone-induced activity of hepatic 7-alkoxycoumarin O-dealkylase in mice.