1. This study investigated the subcellular localization of injected N-acetylneuraminic acid (NeuNAc) in brain. Forty pregnant rats were distributed into four groups. Two groups were given a 200 g casein/kg diet and the other two groups a 100 g casein/kg diet throughout gestation. One group from each of the low- and high-protein groups were given their respective diets for the first 11 d of lactation. On day 12 of lactation, 2.5 microCi NeuNAc/kg body-weight were injected intraperitoneally into their pups. After 1 h the pups were killed, their brains removed and subjected to subcellular fractionation. On day 16 of lactation the other two groups were similarly treated. 2. In all groups of animals 80% of the [14C]NeuNAc incorporated into the brains was found in the synaptosomal fraction and the remainder distributed among the other subcellular fractions in proportion to their total NeuNAc content. 3. These results suggest that NeuNAc exerts its effects on behaviour via the synaptic membrane.