N-methyl-D-aspartate (NMDA) receptor activation, at the level of the spinal cord, has been shown to play an important role in the facilitation of nociception in several animal models. However, the use of NMDA antagonists as analgesics is limited by serious side effects due to nonselective effects among the NMDA receptor subtypes. Recent discoveries revealed that the transfection of small interfering RNAs (siRNAs) into animal cells resulted in the potent, long-lasting, post-transcriptional silencing of specific genes. Thus, we investigated the effect of intrathecal (i.t.) injection of siRNAs targeting NMDA-R2B receptor subunit protein (NR2B) receptors, a subunit of NMDA receptor, for the modulation of pain. The results indicate that the use of siRNA targeting the NR2B subunit not only decreased the expression of NR2B mRNA and its associated protein, as demonstrated by real-time PCR and Western blotting, but also abolished formalin-induced pain behaviors in rat model. The peak effect occurred on day 3 for mRNA and day 7 for its protein, following i.t. injection of 5 microg of siRNA-NR2B. These data prove the feasibility of i.t. siRNAs in the investigation of functional gene expression in the context of whole animal behavior for the management of chronic pain.