INTRODUCTION: Breast cancer is the most common type of cancer affecting women worldwide. Among the treatments, radiation therapy (RT) is frequently chosen as a primary strategy; however, it demands high doses of ionizing radiation to achieve a curative dose. To enhance RT effectiveness, an external agent can be used to sensitize cells before the treatment, allowing a dose reduction. Nitric oxide (NO) is an essential molecule linked to several organic processes, besides being described as a potential radiosensitizer of tumor cells by different mechanisms, including oxidative stress. However, NO have a short half-life in biological conditions, making it difficult to achieve anticancer effects. To overcome this, NO donors can be encapsulated into polymer-based nanoparticles, ensuring a sustained NO releasing. OBJECTIVES: To evaluate the cytotoxicity induced by chitosan nanoparticles containing S-nitrosoglutathione (GSNO-CS NPs) in 4T1 cells (murine triple-negative breast cancer). MATERIALS AND METHODS: Cells were cultivated, seeded in 96-well plates (2 x 10 4 cells/well), and incubated at 37??C with 5% of CO2 for 24 h. Both CS NPs and CS NPs containing GSNO encapsulated were added to the plates at different concentrations (0-2.4 mg/ml CS NPs, 0-6 mM GSNO) and incubated for 24 h. Cytotoxic effects were evaluated through Resazurin fluorometric assay in both groups. DISCUSSION AND RESULTS: Our results showed a 65% reduction in cell viability for GSNO-CS NPs groups treated at 6 mM, while only 30% of cells were killed when treated by CS NPs group. CONCLUSION: Our data suggest that GSNO-CS NPs were able to promote cytotoxicity effects, thus inducing oxidative stress in triple-negative breast cancer cells. Next steps involve the use of these nanoparticles before RT to evaluate its radiosensitizer effect.