Flavonoids have been recognized as the active ingredients of many medicinal plant extracts due to interactions with proteins via phenolic groups and low toxicity. Here, we report the investigation of the flavonoid core as a potential new scaffold for the development of opioid receptor ligands. Biological results suggest that stereochemistry of the C2 and C3 positions is important for antagonist activity and selectivity. Our results also suggest that the actions of Hypericum perforatum may be mediated in part by opioid receptors.