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High copper (Cu) accumulation in medicinal plants might favor lipid peroxidation and hence affect antioxidant responses. This effect was studied by determining antioxidant activities in the water extracts of medicinal plants from Cu mining impact site and compared with control samples. Antioxidant activities of the extracted medicinal plants were assayed by measuring (1) total phenolic and flavonoid contents, (2) diphenyl-1-picrylhydrazyl (DPPH)• free radical scavenging activity, and (3) inhibition capacities of lipid peroxidation. The total phenolic as well as total flavonoid contents of the mining impact samples and control samples are not significantly different. However, inhibition of lipid peroxidation capacities was significantly less (12-60%) for mining impact samples as reflected by the IC50 values. These anomalies were attributed to high concentrations of Cu (2-4-fold higher) in mining impact samples as measured by inductively coupled plasma mass spectrometry. A positive correlation was observed between Cu levels and IC50 values for the inhibition of lipid peroxidation for mining impact samples. [ABSTRACT FROM AUTHOR]