Whether preformed dietary docosahexaenoic acid (DHA) is required for brain accretion has not been clearly determined. In this study, we investigated in mice the different effects of dietary longer-chain n-3 polyunsaturated fatty acids (PUFAs) and α-linolenic acid (LNA) on brain accretion of DHA and the expression of associated desaturases and transcription factors. C57 BL/6J mice were fed for 3 months with four fish oil n-3 PUFA diets--lower, low, high and higher (0.46%, 0.91%, 1.73% and 4.29% total energy, respectively); a flaxseed oil n-3 PUFA (5.01% total energy) diet; and an n-3 PUFA-deficient diet, respectively. Either fish oil or flaxseed oil n-3 PUFA diets increased DHA concentrations in the brain. However, the flaxseed oil n-3 PUFA diet was less effective than the fish oil diet with higher amount of n-3 PUFA in increasing brain DHA content. Furthermore, the expressions of delta-6 desaturase (D6D) and sterol regulatory element binding protein 1 (SREBP-1) in the liver were down-regulated by all fish oil diets with different amounts of n-3 PUFAs, as well as by the flaxseed oil n-3 PUFA diet, whereas in the brain, D6D, delta-5 desaturase (D5D) and SREBP-1 expressions were down-regulated by the higher fish oil n-3 PUFA diet rather than by other fish oil n-3 PUFA and the flaxseed oil n-3 PUFA diets. These results suggest that preformed dietary DHA, different from those converted by LNA inside the body, is better for brain accretion. Dietary longer-chain n-3 PUFAs affect expressions of D6D, D5D and SREBP-1 in the brain differently from their precursor LNA.