Twenty-two species of medicinal plants collected in the Mexican state of Morelos were selected to evaluate their free radical scavenging and antioxidant activities. The extracts from the aerial parts of the plants were obtained using hexane, acetone and methanol (66 extracts). The initial qualitative screening of antioxidants was made using two TLC methods against the stable DPPH (1,1-diphenyl-2- picrylhydrazyl-hydrate) and -carotene-linoleic acid bleaching assay. All the extracts displayed antioxidant activity. However, the methanol extracts appeared to have the highest antioxidant activity, so they were examined further. The quantitative assays against DPPH radical, -carotene-linoleic acid bleaching and total amount of phenols in the methanol extracts (TPME) showed nine plants as having the highest scavenging and antioxidant activities. For the nine methanolic extracts analysed, a clear relation between the total phenolic content of the extracts and their antioxidant activity was found. Plants such as Annona squamosa and Sapium macrocarpum showed two times more antioxidant activity than the commercial BHA (butylated hydroxyanisole) antioxidant. Moreover, some methanolic extracts of the plants showed activities comparable to commercial antioxidants BHA and TBHQ (tertbutylhydroquinone), thus making it possible to consider some of the studied plants as a potential source of antioxidants of natural origin.