The aim of this study was to evaluate the effects of a fish oil (FO) diet (rich in long chain, n-3, polyunsaturated fatty acid) on cancer cachexia symptoms in rats. To this end, peritoneal carcinosis (PC) was generated by an intraperitoneal injection of cancer cells in BDIX rats fed FO or standard (Std) diets. Food intake and body weight were recorded throughout the study until sacrifice. PC rats were sacrificed when food intake was significantly and severely reduced. Fat and skeletal muscles masses were weighed and serum inflammatory cytokines concentration measured at sacrifice. Occurrence of anorexia in PC rats was delayed in the FO diet group (median time was multiplied by 2.5) in comparison with Std diet. At the time of sacrifice, PC rats displayed a lower body weight gain as well as lower muscle and fat masses than pair-fed rats, suggesting the presence of a hypermetabolism state. Serum TNF-alpha was significantly increased in PC rats compared with controls rats. There was no effect of FO diet on tissue mass (skeletal muscle and fat) or on TNF-alpha concentration. In conclusion, FO diet delays the appearance of anorexia induced by PC in rats.