Although most proteins undergo turnover, a few extremely long-lived proteins (ELLPs) have been identified. To detect potential ELLPs in the rat brain, the authors fed rat pups a 15N-enriched diet until they were 6 weeks of age and a 14N-enriched diet thereafter. Twenty-five nuclear proteins, mainly histones and nuclear pore complex (NPC) proteins, from brains of rats sacrificed at 12 months had a high 15N/14N ratio, suggesting that they were ELLPs. The lack of protein turnover in brain NPCs may underlie the age-related decline in NPC function in rats.