In this article we demonstrate the potential of herbal extracts from yerba mate (Ilex paraguayensis) as an oral contrast agent for MRI. At typical drinking concentrations, yerba mate acts as a “biphasic” contrast agent with T1 weighting at short echo times and T2 weighting at echo times greater than about 40 ms. Based on data obtained from X-ray fluorescence elemental analysis, NMR relaxometry, and ESR we identify the relaxation agent in the extract as a low-molecular-weight manganese complex. Yerba mate exhibits an unusually high manganese content that is readily available for hot water extraction. Despite the high elemental manganese levels in I. paraguayensis extract, no manganese-related toxicity of yerba mate has been observed even among heavy yerba mate drinkers, indicating that the manganese in the extract has only a very low bioavailability. Imaging results on staff and patient volunteers demonstrate good contrasting of the GI tract. The relaxation studies of the contrast agent show a sensitivity to pH that is consistent with imaging results from stomach and small bowel. Magn Reson Med, 2006. © 2006 Wiley-Liss, Inc.