Eight-month-old male Sprague Dawley rats were fed diets containing ␣-tocopherol (Toc) or tocotrienol (T3) mixture (composed of 20.5% ␣-Toc, 21.4% ␣-T3, 36.5% ␥-T3 and other analogs) at the 0.1 or 0.5% level for 3 weeks to examine their dietary effects on lipid metabolism and immune indices. Feeding of ␣-Toc and T3 significantly decreased liver phosphatidylcholine peroxide and serum phospholipid levels. In the regulation of immunoglobulin level, significant increase in serum IgA level was observed in the rats fed ␣-Toc or T3, but the effect on immunoglobulin productivity from spleen and mesenteric lymph node (MLN) was not as marked. Feeding of ␣-Toc and T3 significantly decreased LTB 4 -releasing activity of peritoneal exudate cells without decreasing arachidonic acid level. The suppression of LTB 4 release was more marked in the rats fed 0.1% ␣-Toc or T3 than in these fed them at the 0.5% level. When animals were killed after 10 h of fasting, T3 was detected only in MLN and epidydimal adipose tissue. These results suggest that T3 modulates lipid metabolism and immune functions as well as ␣-Toc, and that MLN and adipose tissue are the main target tissues of T3.