Endogenous generation of prostacyclin (PGI2) and lipid peroxidation were studied in the aorta of rats fed on vitamin E-deficient diet and/or vitamin E-supplemented one for 4 to 10 months after they were weaned at 4 weeks. PGI2 was produced by the incubation of the aortic rings in pH 9.0 borate-buffered saline and was estimated by comparison of its anti-aggregatory activity with that produced by known amounts of synthetic PGI2. Thiobarbituric acid-reactive substance (TBARS) was determined as an indicator of lipid peroxidation. 15-Hydroperoxyarachidonic acid inhibited PGI2 generation in vitro. The generation was significantly reduced in rats fed on vitamin E-deficient diet for 8 and 10 months as compared with that in the animals fed on vitamin E-supplemented one for the same period (p<0.001). Mean concentration of TBARS in the aortae of rats fed on vitamin E-deficient diet for 10 months was significantly higher than that of the animals fed on vitamin E-supplemented diet for the same feeding period (p<0.001). These alterations in the aortae of rats fed on vitamin E-deficient diet were corrected by feeding them on vitamin E-supplemented one for subsequent 2 months.