Heavy metal-contaminated sites are excellent areas to examine the antioxidative machinery responsible for physiological adaptations of many plant species. Superoxide dismutase (SOD), guaiacol peroxide (GPX), ascorbate peroxide (APX), catalase (CAT) activity and hydrogen peroxide (H(2)O(2)) content were analyzed in leaves and roots of Viola tricolor (Viola) from contaminated soils ('Bukowno', 'Saturn', 'Warpie' heaps), and non-contaminated soil ('Zakopane meadow') to examine the level of oxidative stress and antioxidative response. In leaves, six isoforms of SOD were recognized. Roots possessed two additional bands, named manganese superoxide dismutase (MnSOD)-like form (MnSODI) and Cu/ZnSOD-like form (Cu/ZnSODIV). The H(2)O(2) content in leaves ranged from 554 to 5 098 micromol H(2)O(2)/gf.w. and was negatively correlated with CAT activity. The non-contaminated population was characterized by the lowest CAT activity combined with the highest H(2)O(2) concentration. Two isoforms of CAT, CAT-1 and CAT-2, were recognized in leaves of plants from non-contaminated and contaminated sites, respectively. In roots of individuals from two heaps ('Warpie' and 'Saturn'), two distinct bands for each CAT isoform were observed. A slower migrating band may be an aggregate, exhibiting CAT and MnSODs activities. Both peroxidases (APX and GPX) presented the same pattern of activity, depending on the organ, indicating that in leaves and roots APX and GPX were regulated in parallel. Differences in enzyme activities and H(2)O(2) content between plants from different contaminated sites were statistically significant, but were tightly maintained at a very similar level. Prolonged and permanent heavy metal stress evoked a very similar mode of antioxidative response in specimens of analyzed metalliferous populations not causing measurable oxidative stress. Thus, our results clearly indicate that V. tricolor is a taxon well adapted to heavy metal-contaminated soils, and that differences in enzyme activities and H(2)O(2) content result from adjustment of plants to a variety of conditions.