Types of dietary lipid affect the life span of rats. In this study. we investigated the influence of the life-long dietary n-6/n-3 ratio on life span and serum lipid and glucose levels. A semi-purified diet adjusted to a constant saturated : monounsaturated : polyunsaturated fatty acid ratio and an n-6/n-3 ratio of 1. (R1), 4 (R4) or 16 (R16) was fed to rats (n=33) from 4 wk of age until death. There were no significant differences in the food intake or body weight, nor were there survival curve or mean life span variations among the 3 groups. The serum cholesterol levels after feeding the test diet for 6 and 12 mo were significantly lower in the R1. group than in the other groups, and the serum triacylglycerol levels were significantly lower than those in the R16 group. However, no significant differences were noted in the serum cholesterol or triacylglycerol level after feeding for 18 mo among the 3 groups. A significantly higher serum glucose level was noted in the RI group only at 18 mo of test diet ingestion, compared to that in the R4 group. The results suggest that the influence of the dietary n-6/n-3 ratio on the serum lipid and glucose levels varies, depending on the duration and life stage of feeding. Our findings further suggest that the life span of Wistar rats is not affected even if the ratio of dietary n-6/n-3 changes from 1 to 16.