Neuroserpin is a serine protease inhibitor expressed in the developing and the adult nervous system. Studies with genetically modified mice indicate a role of neuroserpin in the regulation of anxiety. Mutations in the neuroserpin gene cause protein polymerization and formation of inclusion bodies leading to progressive myoclonic epilepsy and neurodegeneration. Here we demonstrate that neuroserpin expression is regulated by thyroid hormone (T3). Neuroserpin RNA levels are down-regulated in cortical layers II/III and VIa, the hippocampus, the retrosplenial cortex and the medial habenular nucleus, but not in cortical layer V or other areas of the hypothyroid rat brain. Concordantly, neuroserpin RNA and protein expression was induced by T3 in rat PC12 cells containing appropriate thyroid hormone receptor levels. In run-on assays T3 did not affect the transcription rate of the neuroserpin gene, indicating that regulation was post-transcriptional. Moreover, T3 increased in vitro binding of cytoplasmic proteins to neuroserpin 3'-UTR RNA and caused biphasic regulation of the stability of this transcript in PC12 cells. Ectopic neuroserpin expression induced neurite extension in PC12 cells and enhanced neuritogenesis triggered by nerve growth factor. In summary, these results indicate that neuroserpin expression is post-transcriptionally regulated by T3 at the level of RNA stability.