The rate of desaturation of linoleic acid (18:2(n - 6)) and level of arachidonic acid (20: 4(n - 6)) in mucosal microsomes from small intestine of rats fasted for 24 h or fed diets of different fatty acid composition was examined. Fasting or feeding a diet high in linoleic acid increased delta 6-desaturase activity, a rate-limiting enzyme in the arachidonic acid biosynthetic pathway in the jejunum. After fasting, delta 6-desaturase activity was also enhanced in the ileum. Feeding a diet rich in n - 3 fatty acids had no significant effect on delta 6-desaturase activity in jejunal or ileal mucosal microsomes. Following fasting, arachidonic acid content of microsomal total phospholipids increased in the jejunum with a concomitant decrease in linoleic acid content. Arachidonic acid and 18:2(n - 6) concentration remained unchanged in ileal microsomes after short-term food withdrawal. Feeding a diet containing n - 3 fatty acids lowered the content of 20:4(n - 6) and increased 20:5(n - 3) and 22:6(n - 3) levels in both jejunal and ileal microsomes. These data indicate that the level of 20:4(n - 6) and the biosynthesis of 20:4(n - 6) by desaturation-chain elongation of 18:2(n - 6) in the rat enterocyte responds rapidly to change in physiological conditions such as fasting and dietary fat composition.