To investigate effects of dietary fish oil and vitamin E level on the lipid composition and stability of membranes, male Sprague-Dawley rats were fed experimental diets composed of either menhaden oil or soybean oil and either a high (equivalent to 140 mg ${alpha}-tocopherol$) or a low (equivalent to 17 mg ${alpha}-tocopherol$) vitamin E level for 4 weeks. The fatty acid compositions of liver microsomes reflected those of dietary lipids without effect of vitamin E. The cholesterol/phospholipid molar ratio was elevated in the groups fed fish oil than soybean oil. ${alpha}-tocopherol$ level of microsomes was significantly lowered not only by low dietary vitamin E level but also by fish oil feeding. Levels of lipid peroxides (thiobarbituric acid-reactive substances) of microsomes tended to increase by low vitamin E level and by fish oil feeding. The levels of lipid peroxides were independent of dietary factors after in vitro lipid peroxidation by $NADPH-Fe^{2+}$ for 30 min which resulted in a 4-fold increase. Activities of microsomal enzymes, glucose-6-phosphatase and UDP-glucuronyl transferase, and steady-state fluorescence anisotropy were not influenced by either dietary lipid or dietary vitamin E level. The in vitro lipid peroxidation reduced the activity of glucose-6-phosphatase by 60% and increased the fluorescence anisotropy from 0.135 to 0.156, which would indicate the decrease of membrane fluidity. However, no significant difference was found among the groups. The present work indicates that either fish oil as compared to soybean oil or the low vitamin E level in diets would render microsomal membranes more susceptible to oxidative stress but, in spite of increased oxidative stress, the stability of liver microsomal membranes was hardly affected by the dietary treatment.