Diseases caused by plant parasitic nematodes (PPNs) are still a heavy constraint on modern farming. They can cause losses in crop yield and increase production costs, mainly due to pest management activities. The root lesion nematodes (RLNs), Pratylenchus (Nematoda: Pratylenchidae), are soil-dwelling migratory endoparasites that infect the roots of many crop species. Affected plants develop necrotic spots where the RLNs feed and reproduce in the cortical cells. Injury to plant tissue results in weakened plants that become more prone to attack from opportunistic pathogenic microorganisms. In alfalfa, (Medicago sativa L.), resistance to Pratylenchus penetrans has been linked to an increase in transcripts of key enzymes in the biosynthesis of isoflavonoid phytoalexins; however, the mechanisms of resistance are still unknown. The present work analyzed the activity of antioxidant enzymes of extracts from transgenic roots of susceptible (cv. Baker) and resistant (cv. MNGRN-16) alfalfa. Transgenic roots were obtained after infection of aseptic germinants with Rhizobium rhizogenes and maintained in Schenk and Hildebrandt culture medium. The activity of guaiacol peroxidase (GPX) was ca. 3x higher on extracts of susceptible alfalfa in comparison with those of resistant alfalfa transgenic roots. This suggests that resistance response is not solely dependent on the antioxidant enzymatic activity of GPX. Future work will focus on the metabolomic characterization of these varieties in contact with the RLN.