This study evaluated both the antioxidant activity of Menthae longifoliae folium and the influence of Topsin M (a pesticide which is widely used nowadays) upon the antioxidant activity of the drug. In this respect, there have been used leaves of Mentha longifolia (L.) Huds., both untreated and treated with Topsin M. The leaves were dried, powdered and extracted with aqueous methanol. Taking into account the fact that polyphenols, and especially flavonoids act as antioxidants, these compounds were quantified in the crude extracts. Both the capacity of inhibition of 15-lipoxygenase and the diphenylpicrylhydrazyl radical-scavenging capacity were also evaluated. The undergone study highlighted the fact that crude extracts of Menthae longifoliae folium had a good antioxidant activity; the extracts were active both as diphenylpicrylhydrazyl radical scavengers and as 15-lipoxygenase inhibitors. Treatment with Topsin M did not consistently influence the antioxidant activity of the drug.