Docosahexaenoic acid (DHA) may have potential anticarcinogenic effect. In the present study, effect of DHA on rat C6 glioma was tested. In vitro, cytotoxic effect of 50-400 microM DHA on C6 cells was evaluated and compared with linoleic acid (LA). In vivo, adult female Wistar rats implanted with C6 tumor, fed 1 ml of DHA oil (containing 73% DHA, 36 rats) or LA oil (containing 72-77% LA, 41 rats) daily, starting one week prior to tumor implantation until death or if survived, until 30 days after implantation. Another group of tumor bearing rats was treated with chloroethyl-cyclohexyl-nitrosourea (CCNU, 30 mg/kg, 31 rats) at day 8 post implantation to show if the result of oil supplementation is comparable to single agent chemotherapy. mRNA expression of p21 and p27 was determined in vitro at 100 and 150 microM of fatty acids and in tumors of rats supplemented with LA or DHA oils. In vitro, DHA, but not LA, had cytotoxic effect on C6 cells at 200 and 400 microM and DHA increased mRNA expression of p21 at 150 microM (p < 0.05). In rat glioma model, although a non-significant trend towards better survival was observed in DHA oil relative to LA oil group, the difference was not significant (p = 0.20). p21 and p27 mRNA expression in tumors of DHA oil group did not differ with LA oil group. Single dose of CCNU increased survival when compared to LA oil group (p < 0.001). In conclusion, intake of DHA at the dose or duration employed in the present study might be insufficient to bring about its cytotoxic action on rat's C6 brain tumor.