Teas made from medicinal plants are commonly used by a majority of the inhabitants of New Mexico and Mexico to treat various ailments including infections, arthritis, heart disorders, headaches, fever, asthma and menstrual pain. However, little is known about the identity or chemical nature of the bioactive substances and compounds responsible for the therapeutic effects of the teas made from the leaves, seeds, flowers stems, and roots of these medicinal plants. Some of the beneficial effects of these teas may be attributable to antioxidants contained in the medicinal plants from which they are brewed. In the present study we collected 30 medicinal plants that are widely used in the Rio Grande Valley and, using a two-stage Trolox based assay, analyzed the total antioxidant capacity of aqueous extracts prepared from these plants. The antioxidant content of the aqueous extracts was substantial, ranging from 27 to 972 micromol Trolox equivalent per gram dry weight. An extract of the leaves of the plant Ilex paraguensis (Mate leaf) contained the highest amount of antioxidant, followed by the flowers of the Rosa sp. (Rosa de Castillo, 804 micromol/g), the bark of Chinchona sp. (Copalquin, 692 micromol/g), Rumex hymenosepalus stems (Cana Agria, 672 micromol/g) and the leaves of Marrubium vulgare (Mastranzo, 560 micromol/g). The plants that had the lowest antioxidant capacity were the seeds of Linum lewissii (Linasa, 29 micromol/g) and Yucca sp. plant root (Amole, 27 micromol/g). It will be useful to further analyze those plants that contain the most antioxidant activity in order to identify the active principles.