The optimal dietary n-6/n-3 ratio has not been fully elucidated. To investigate the influence of the dietary n-6/n-3 ratio on this ratio in the body and on liver beta-oxidation peroxisomal activity, rats were fed diets containing fat at an n-6/n-3 ratio of 1 to 16 for 4 weeks. To investigate whether elevation of the liver peroxisomal beta-oxidation activity increases the n-6/n-3 ratio in the body, rats were fed a diet containing a peroxisome-activating agent, bezafibrate, for 2 weeks, and its influence on the liver n-6/n-3 ratio was examined. The slope of the regression line between the dietary and liver total lipid n-6/n-3 ratios was significantly smaller when the dietary n-6/n-3 ratio was 4 or greater than when it was smaller than 4. Peroxisomal beta-oxidation and acyl CoA oxidase activities were significantly lower in rats fed a diet with an n-6/n-3 ratio of 16 than in those fed a diet with a ratio of 1. The peroxisomal beta-oxidation activity in the bezafibrate-supplemented group was significantly higher than that in the control group. The serum and liver total lipid n-6/n-3 ratios were significantly higher in the 0.015% bezafibrate-supplemented group than in the control group. These findings suggest that the liver n-6/n-3 ratio might be controlled via peroxisomal beta-oxidation in rats.