Daily administration of either phenobarbital (60 mg/kg) or yin zhi huang (YZH,30-60 ml/kg) for 5 days to rats similarly accelerated the clearance and conjugation of intravenously infused bilirubin. However, the two drugs had quite different effects on liver enzyme activities associated with xenobiotic metabolism and with bilirubin metabolism. Phenobarbital markedly increased cytochrome P-450 levels and the cytochrome-P-450-mediated formation of 4-hydroxybiphenyl whereas YZH had a slightly depressive effect on this enzyme system. Both drugs increased glucuronyl transferase activity using bilirubin andα-naphthol as substrates. Bilirubin conjugation in microsomes activated by uridine diphosphate-N-acetylglucosamine (NAG) was greater in YZH- than in phenobarbital-treated rats whereas phenobarbital was a more effective inducer in digitonin-activated enzyme. Whenα-naphthol was used as substrate (NAG-activated glucuronyl transferase), pretreatment with phenobarbital produced a greater increase in activity than did YZH. Glutathione-S-transferase activity (using chlorodinitrobenzene as substrate) was increased more than 2-fold by phenobarbital but only slightly (1.29 ×) by YZH. In contrast, YZH (60 ml/kg) was more effective than phénobarbital in increasing glutathione peroxidase activity using cumene hydroperoxide as substrate. YZH appears to be a relatively specific inducer of enzymes involved in bilirubin metabolism.