We previously reported the effect of St John’s wort (Hypericum perforatum) standardised extract LI 160 on the activities of cytochrome P450 2C6, 2D2 and 3As (Dostalek et al., Life Sciences 2005;78(3):239-44). In this study, we aimed to assess the effect of St John’s wort on the activity of cytochrome P450 1A2. The isolated perfused rat liver model was used in our experiments with phenacetin as a marker substrate for cytochrome P450 1A2. The rate of formation of acetaminophen was significantly inhibited by the use of St John’s wort. In addition, St John‘s wort extract inhibited cytochrome P450 1A2 activity significantly more than the control inhibitor. St John’s wort significantly inhibited cytochrome P450 1A2 enzyme activity. It remains to be determined whether the co-administration of St John‘s wort extract and other medications or substrates for cytochrome P450 1A2 could result in significant drug interactions.