Response of four improved grass pea (Lathyrus sativus L.) genotypes to arsenic was tested in pot experiment using 30 mg As l-1 up to 60 days of growth after commencement of treatment (DAC). Arsenic exposure significantly reduced growth and seed yield potential of vars B1 and BioL-212, but no such effect was observed in bold-seeded mutant and pod indehiscent mutant lines. Results revealed normal leaf photosynthesis and antioxidant metabolism at 20 DAC in both varieties. However, high superoxide dismutase activity coupled with low ascorbate redox and declining ascorbate peroxidase level led to abnormal rise in H2O2 content at reproductive stages (40 and 60 DAC), consequently, resulting in significantly enhanced arsenic-induced oxidative damage and physiological impediment in both varieties. By contrast, H2O2 level in both the mutants was remarkably modulated at reproductive stage (40 DAC) in a highly controlled manner by balanced action of antioxidant defense. This favored normal photosynthesis and ensured good grain yield even under prolonged arsenic exposure by effectively preventing oxidative damage to membrane in both the mutants. Native PAGE analysis revealed that enzyme isoforms were involved in the regulation of foliar H2O2 metabolism, the level of which was found extremely critical in determining arsenic tolerance of grass pea genotypes. © 2013 NAAS (National Academy of Agricultural Sciences).