Polyunsaturated fatty acids are avidly taken up by the developing rat brain. To explore the specificity of this process, [1-14C]labeled 16:0, 18:2n-6, 18:3n-3, and 22:6n-3 each were co-injected with [3H]18:1n-9 into the jugular vein of two-wk-old functionally hepatectomized and sham-operated control rats. The radioactivities present in the brain, liver and serum were assessed 30 min after injection. Uptake of labeled fatty acids into brain lipids steadily increased with increasing degree of unsaturation, with more than twice as much uptake of 22:6n-3 compared to 16:0. Phosphatidylcholine was the principal radioactive species in the brain except for animals injected with [1-14C]22:6n-3, in which more of the label was incorporated into phosphatidylethanolamine. Determination of water-soluble oxidation products in the brain and serum revealed that the greater uptake of the more unsatrated fatty acids did not result from differences in rates of degradation.