The concentration of cytochrome P450, measured spectrophotometrically in microsomal preparations from the livers of rats infected with 30 metacercariae of Fasciola hepatica, declined by approximately 50% at 3 weeks post-infection. Treatment of infected rats with the anti-inflammatory agent dexamethasone (2 mg/kg at 48 h intervals for 8 days prior to assay) abolished the decline in P450 content. Assay of P450 in infected congenitally athymic (nude) rats showed normal levels. These results demonstrate that the T-cell-dependent inflammatory response in the liver of the host is a necessary factor in the development of the decline in hepatic P450, which is known to compromise the metabolism of certain drugs in infected hosts.