ABSTRACT Capirona decorticans (Rubiaceae) is popularly used to treat warts, wounds, mycoses and scabies, and is also a component of the Ayahuasca tea. Despite its popular use, the phytochemical and pharmacological research on this species is limited. Therefore, this work quantified phenolic compounds in the ethanolic extract (EE) and hydromethanolic fraction (FM) (406, 293 mgEAG g-1, respectively) from leaves of C. decorticans. We identified flavonoids by LC-MS/MS-MMR-ESI (apigenin, rutin, luteolin, miricetin, quercetin, quercetin-3-β-D-glucoside, quercetrin), and evaluated oxidative stress and mutagenic/antimutagenic effect of EE and FM through an in vivo experiment using Swiss mice and cyclophosphamide (CP) as an inducer of DNA damage and oxidative stress. Mice were pretreated for 15 consecutive days with EE or FM (250 mg kg-1) and then intraperitoneally injected with CP (25 mg kg-1). Carbonylated proteins, ascorbic acid, catalase and thiobarbituric acid-reactive substances were measured in hepatic and renal tissues. The mutagenic/antimutagenic effect was evaluated through the Micronucleus Test. Protein carbonylation in the liver of animals exposed to CP was reduced by FM. There was no significant effect on other markers of oxidative stress. The groups treated with the extracts showed a significant percentage reduction (EE = 96% and FM = 71%) in the frequency of micronucleated polychromatic erythrocytes induced by CP. EE showed mutagenicity when used alone. The EE and FM of C. decorticans leaves showed antioxidant potential equivalent to that observed in other species, did not cause oxidative stress, nor toxicity, and had a protective and antimutagenic effect, although the EE showed signs of mutagenicity.