We investigated the effect of fish oil (FO) supplementation on tumor growth, cyclooxygenase 2 (COX-2), peroxisome proliferator-activated receptor gamma (PPARγ), and RelA gene and protein expression in Walker 256 tumor-bearing rats. Male Wistar rats (70 days old) were fed with regular chow (group W) or chow supplemented with 1 g/kg body weight FO daily (group WFO) until they reached 100 days of age. Both groups were then inoculated with a suspension of Walker 256 ascitic tumor cells (3 × 10(7) cells/mL). After 14 days the rats were killed, total RNA was isolated from the tumor tissue, and relative mRNA expression was measured using the 2(-ΔΔCT) method. FO significantly decreased tumor growth (W=13.18 ± 1.58 vs WFO=5.40 ± 0.88 g, P<0.05). FO supplementation also resulted in a significant decrease in COX-2 (W=100.1 ± 1.62 vs WFO=59.39 ± 5.53, P<0.001) and PPARγ (W=100.4 ± 1.04 vs WFO=88.22 ± 1.46, P<0.05) protein expression. Relative mRNA expression was W=1.06 ± 0.022 vs WFO=0.31 ± 0.04 (P<0.001) for COX-2, W=1.08 ± 0.02 vs WFO=0.52 ± 0.08 (P<0.001) for PPARγ, and W=1.04 ± 0.02 vs WFO=0.82 ± 0.04 (P<0.05) for RelA. FO reduced tumor growth by attenuating inflammatory gene expression associated with carcinogenesis.