Organic anion transporters (OATs in humans, Oats in rodents) play an important role in the distribution and excretion of numerous endogenous metabolic products and exogenous organic anions, including a host of widely prescribed drugs. Their ligand recognition is also important for drug therapy and development. In this study, the n-butanol and dichloromethane soluble fractions of Juniperus oblonga were found to inhibit OAT3 in vitro and three biflavonoids were found to be responsible for this activity. One of these compounds, amentoflavone exhibited stronger inhibition than probenecid, a known strong inhibitor of OAT3. Biological characterization of amentoflavone in vivo also showed inhibition of Oat3. Preliminary observations of structure-activity relationships suggest that the biflavonoids are more potent inhibitors of this transporter than their corresponding monomer, and that methylation of even a single hydroxyl group results in a substantial decrease in activity. This greater potency of the biflavonoids may indicate the need for a more in-depth investigation of the distribution of biflavonoids in plants used as foodstuffs and herbal medicines, due to their potential for causing interactions with OAT3 substrate drugs. (C) 2019 Elsevier Inc. All rights reserved.