The dietary bioequivalence during the brain growth spurt ofα-linolenic (LNA) and docosahexaenoic acids (DHA) as substrates for brain and retinaln-3 fatty acid accretion is reported for the fetal baboons, whose mothers consumed a long chain polyunsaturate-free diet with an-6/n-3 ratio of 10:1. Pregnant baboons received i.v. doses of U-13C-labeled fatty acids (LNA or DHA), plasma was collected from mother and fetus, and fetal brain (occipital cortex), retina, and liver were analyzed at various times post-dose. Fetal brain DHA plateaued 15-35 d post-dose with 1.6% of the preformed [U-13C-]DHA dose recovered in the brain. In contrast, LNA-derived DHA accretion also plateaued but was 20-fold lower. Liver and retinal results were of the same order of magnitude, but showed evidence of peaks and decline. Conversion products ton-3 long chain polyunsaturate were observed in the maternal circulation at 1 h after administration, as was transfer of both fatty acids to the fetus. From these measurements we estimate that a dietary level of about 0.45% of energy as LNA is sufficient to meet the requirements of the growing fetal brain, whereas 0.03% of energy as DHA would suffice. These data are the first direct measurements of the bioequivalence of DHA and LNA in developing primates and imply thatn-3 fatty acid requirements for the developing fetal brain can be met by attainable dietary LNA for diets low in long chain polyunsaturates.