The aim of this study was to observe whether the dietary supplementation of docosahexaenoic acid(DHA). In growing rats requires extra supplementation of arachidonic acid(AA) for brain development. Sprague-Dawley rats were divided into three groups, each fed a different diet. In the FO group, dams were fed a DHA-rich FO diet during pregnancy and lactation and pups were fed the same diet until 10 weeks old. In the AO group dams and pups were similarly fed a FO diet after weaning. DHA and AA were most effetively deposited in the developing brain during pregnancy and lactation in rats. However, FO-W pups showed significantly lower level of DHA at 0-3 weeks compared with the FO and AO groups and than slowly increased DHA levels to about 87% of other groups at 10 weeks with the introduction of the FO diet after weaning. The total amount of DNA in whole brain rapidly reached a maximum level at 3 weeks and then was sustained at a constant level after 5 weeks of age. The DNA content was positively correlated with DHA level but not with AA level in the developing brain. DNA content was significantly lower in the FO-W group compared to the FO and AO group at 3 weeks of age. However, the DNA content of brain in FO-W pups increased to 80% of the FO group level at 10 weeks after feeding the FO diet after weaning. The relative percentage of AA in brain lipids was significantly reduced in the early stage of brain development when only DHA was supplemented. However, DHA supplementation had no significant effect on the incorporation of AA when the approximately 35% of LA in the FO diet was substituted by preformed AA. These results suggest that large quantities of DHA could interfere with the normal conversion of LA to AA if LA is not supplemented enough together with DHA. Therefore, high DHA supplementation may require preformed AA in the diet even though AA has no significant correlation with the DNA content in brain. DHA supplementation after weaning also improved the incorporation of DHA into brain and content of DNA even though brain development was almost completed, suggesting that a low level of DHA supplementation without AA addition might be necessary to improve brain development during infancy as well as during pregnancy and lactation.