Several herbal medicines can modulate in vitro glutathione-S-transferase activities (GST), which are major phase II enzymes of xenobiotic metabolism. Inhibition of these enzymes may result in increased blood concentration of drugs leading to intoxication of the user, while its induction may cause reduction in the concentration of these drugs, leading to treatment failure. Therefore, concomitant use of herbal and allopathic drugs can have unwanted consequences to the health of the individual. In this work we evaluated the effect of herbal extracts on rat liver glutathione-S-transferase activity. Knowledge of interactions between herbal and allopathic drugs can help prevent damages to the health of the users. Crude extracts and teas were obtained from plant species widely consumed as tea in Brazil: Cymbopogon citratus, Lippia alba, Punica granatum, Peumus boldus, Baccharis trimera, Persea americana, Bauhinia forficata, Zingiber officinalis, Phyllanthus niruri and Allium sp. All vegetable samples evaluated showed inhibitory effects equal to or greater than 50%, with P. granatum (pomegranate) extract showing the highest levels of inhibition of in vitro activity of GST.