Several epidemiological studies show that aspirin can act as a chemopreventive agent and decrease the incidences of various cancers including melanoma. In this work, we investigated the in vitro and in vivo efficacy of acetylsalicylic acid (ASA) as an antimelanoma agent in B16-F0 cells and skin B16-F0 melanoma tumor mouse model. Our findings indicate that the IC50 (48 h) for ASA in B16-F0 melanoma cells was 100 μM and that ASA caused a dose- and time-dependent GSH depletion and increase in reactive oxygen species (ROS) formation in B16-F0 melanoma cells. Male C57BL/6 mice were inoculated s.c. with 1 × 10(6) B16-F0 melanoma cells. ASA (80, 100, and 150 mg/kg) was initiated on day 1 or day 7, or day 9 after cell inoculation and continued daily for 13, 7, and 5 days, respectively. Animals were weighed daily and sacrificed on day 13. The tumors were excised and weighed. The animals receiving 13 days of ASA therapy at 80, 100, and 150 mg/kg demonstrated tumor growth inhibition by 1 ± 12%, 19 ± 22%, and 50 ± 29%, respectively. Animals receiving 7 days of therapy at 80, 100, and 150 mg/kg demonstrated tumor growth inhibition by 12 ± 14%, 27 ± 14%, and 40 ± 14%, respectively. No significant tumor growth inhibition was observed with 5 days of therapy. ASA at 100 and 150 mg/kg caused significant tumor growth inhibition in C57BL/6 mice when administered for 13 and 7 days, respectively. The results obtained in this study are consistent with the recent epidemiologically based report that aspirin is associated with lower melanoma risk in humans.