The toxicity of dietary aspirin on growth rate and lipid metabolism was investigated under linoleic acid (LA; 18: 2n-6) deficient conditions. One-week-old chicks were given diets containing 0 or 2% LA with or without 0.4% aspirin, until 4 weeks of age. Growth was severely depressed by dietary aspirin when chicks were given the LA-free diet. The liver was enlarged by both the aspirin and LA deficiency. The aspirin treatment induced a significant increase of 18:0 and arachidonic acid (20: 4n-6) and a decrease of 18: 1n-9 in the liver. In chicks fed LA-free diets, the ratio of 20:3n-9/20: 4n-6, which was used as an indicator of LA deficiency, was suppressed by aspirin treatment. In conclusion, the present results suggest that aspirin toxicity is altered by dietary LA concentrations.