Polymnia sonchifolia (Asteraceae) is an Andine plant cultivated in South America and known in Brazil as "Yacon". The ethanolic extract from its leaves is used in popular medicine as antiinflammatory, antimicrobian, antihelminthic, fungistatic and antiviral. Phythochemical analysis showed compounds such as sequiterphenes lactones and flavonoids. The aim of this work was to evaluate the antioxidant and the antiinflamatory potentials of the crude ethanol extract (PS_EB) and some its fractions. It was done in vitro using these assays: DPPH, HO^· and O₂^•- scavenging activities and protection against lipoperoxidation. The most promising antioxidant agents were also in vivo evaluated using Balb/c mice (n=6; w=20±4g) and CCl₄ (150µL/100g b.w.) as stressor agent and positive control (PC). It was evaluated through the lipoperoxidation content (TBARS), protein carbonilation, reduced glutathione (GSH) level and catalase (CAT) activity. The carrageenan-induced paw edema assay was used to determine the antiinflamatory activity. All tested products exhibited antioxidant potential in vitro where the acetate hexane (3:7) fraction (PS₂) was the most effective extract at all tests followed by PS_EB when they were compared to Rutin. PS₂ , PS_EB and Rutin presented IC₅₀=47.1±3.0, 19.58±0.7 and 12.8±1µg/mL at the DPPH assay; IC₅₀=<0.1, 1.96±0.4 and 1.4±0.2µg/mL at the HO^· scavenging assay; IC₅₀=<32.5, <32.5 and 10.4±0.1µg/mL at the TBARS assay; respectively. PS_EB did not present activity at the O₂^• scavenging assay; PS₂ and Rutin presented IC₅₀=97.0±4.8 and 83.4±0.2µg/mL; respectively. The products increased the GSH levels (PS_EB=5.3±1.3 and PS₂=4.8±1.1mM) and the CAT activity (PS_EB=479.6±52.0 and PS₂=269.9±56.4mmol min^-1 g^-1) when they were compared to PC (GSH=2.1±0.7mM and CAT=341.6±44.3mmol min^-1 g^-1)(p<0.05). They caused protection against lipoperoxidation (PC=185.1±11.0, PS_EB=151.6±25.8, PS₂=144.6±25.4nmolg^-1, p<0.05) and decreased proteic carbonilation (PC=371.61±146.3; PS_EB=43.28±8.27; PS₂=47.02±10.34nmol/mg, p<0.05). The extracts reduced the paw edema (PS_EB=54.8% and PS2=68.5%) related to carrageenan-group. Based on the results it is possible to conclude PS_EB and PS₂ have important antioxidant effect and they presented favorable data at the carrageenan-induced paw edema assay. This data suggests the studied extracts could be considered promising antiinflamatory agents and the activity could be related to their antioxidant potential maybe associated to their capacity of scavenging HO^· free radical.