The metabolism of metronidazole and antipyrine was investigated in freshly isolated hepatocytes from 7 male and 6 female control Wistar rats, 8 males and 5 females pretreated with phenobarbital (PB) and 3 males pretreated with 3-methylcholanthrene (MC). Pretreatment with PB increased the intrinsic clearance (CLi = Vmax/Km) of metronidazole to its acetic acid (MAA) and hydroxy metabolite (HM) 7- and 2.8-fold in the males and 3.2- and 3.0-fold in the females, whereas MC treatment increased the values 9- and 10-fold, respectively (P less than 0.05). The CLi of metronidazole to HM and its glucuronide conjugate was higher in the control and PB treated male than in the corresponding female groups, whereas the rank order was reversed for sulphate formation (P less than 0.05). SKF 525A was a more potent inhibitor of MAA formation than of HM formation, except in the PB treated male group. Pretreatment with MC increased the inhibitory potency of alpha-naphthoflavone and antipyrine toward MAA and HM formation. In male rats PB treatment increased the CLi of antipyrine to 3-hydroxymethyl-(HMAP), nor-(NORAP) and 4-hydroxyantipyrine (OHAP) 2.5-, 2.1- and 4.5-fold, respectively (P less than 0.05). Pretreatment with MC in male and with PB in female rats had no significant effect on antipyrine metabolism. SKF 525A was a more potent inhibitor of HMAP and OHAP formation than of NORAP formation. Treatment with MC increased the inhibitory potency of alpha-naphthoflavone toward the formation of all antipyrine metabolites. Metronidazole increased the formation rate of HMAP, but inhibited the formation of NORAP and OHAP, particularly the latter. The results suggest that the formation of MAA, HM, HMAP, NORAP and OHAP from metronidazole and antipyrine is catalyzed by different cytochrome P-450 isozymes, which may be supplemented or substituted by PB or MC induced species. The involved P-450 isozymes have more or less overlapping substrate and product specificity. Metronidazole appears to be a sensitive probe for detection and identification of PB and MC type induction.