During perinatal development, docosahexaenoic acid (22:6n-3) accumulates extensively in membrane phospholipids of the nervous system. To evaluate the n-3 fatty acid requirements of fetal and suckling rats, we investigated the accumulation of 22:6n-3 in the brain and liver of pup rats from birth to day 14 postpartum when their dams received increasing amounts of dietary 18:3n-3 (from 5 to 800 mg/100 g diet) during the pregnancy-lactation period. The fatty acid composition of brain and liver phospholipids of pups, as well as that of dam's milk, was determined. At birth, 22:6n-3 increased regularly to reach the highest level when the maternal diet contained 800 mg 18:3n-3/100 g. On days 7 and 14 postpartum, brain 22:6n-3 plateaued at a maternal dietary supply of 200 mg/100 g. Docosapentaenoic acid (22:5n-6) had the opposite temporal pattern. The unusually high concentration of eicosapentaenoic acid (20:5n-3) in liver and dam's milk observed at the highest 18:3n-3 intake suggests an excessive dietary supply of this fatty acid. All these data suggest that the n-3 fatty acid requirements of the pregnant rat are around 400 mg 18:3n-3 and those of the lactating rat at 200 mg (i.e., 0.9 and 0.45% of dietary energy, respectively). The values of 18:3n-3 and 22:6n-3 milk content which allowed brain 22:6n-3 to reach a plateau value in suckling pups were 1% of total fatty acids and 0.9% (colostrum) to 0.2% (mature milk), respectively. These levels are similar to those recommended for infant formulas.