The in vitro metabolism of p-nitroanisole, aminopyrine, and aniline by rat liver microsomal monoxygenases were studied in the presence of different polychlorinated biphenyl (PCB) mixtures and some related hydroxybiphenyls. The tested PCB mixtures contained preferably dichloro- (di-CB), tetrachloro- (tetra-CB), or hexachlorobiphenyls (hexa-CB). All PCB were competitive inhibitors of only aminopyrine demethylation by normal microsomes (Ki 22-39 micron). In microsomes of PCB-pretreated rats the aminopyrine demethylation was inhibited noncompetitively by di-CB and hexa-CB whereas tetra-CB remained a competitive inhibitor (Ki 12 micron). Moreover, after PCB pretreatment all PCB were competitive inhibitors of p-nitroanisole demethylation. 2-OH-biphenyl and 4-OH-biphenyl caused competitive inhibition of aminopyrine demethylation and aniline hydroxylation but failed to inhibit p-nitroanisole metabolism by normal microsomes. Chlorinated 4-hydroxybiphenyls inhibited competitively the metabolism of both type I and type II substrates. However, after PCB pretreatment all phenolic compounds caused uncompetitive inhibition of aniline hydroxylation.