Osteopontin (OPN) is a 39 kDa highly acidic cytokine phosphoprotein, encoded by a single gene which is highly conserved in vertebrates. OPN was originally isolated in bone in 1986(1) and has subsequently been isolated in many other tissues, including the kidney, lung, liver and central and peripheral nervous tissue. OPN plays a variety of roles within different cell types, including adhesion, cell survival, wound healing and tumorigenesis. Studies have shown that, within the dorsal root ganglia (DRG), OPN is expressed in 25% of total neurons, and is largely confined to parvalbumen expressing neurons(2). However, to date few studies have analysed its role in the peripheral nervous system. Previous work in our laboratory has demonstrated a sustained 36% upregulation in OPN mRNA within the DRG 1 and 3 weeks after axotomy. This may suggests a role in cell survival and/or the regeneration of damaged neurons. To further elucidate the role played by OPN within the peripheral nervous system a range of experiments has been conducted. Mice were anaesthetised with 1 part Hypnorm and 1 part Hypnovel to 2 parts water. For axotomy, the dose was 150 ? 200ul I.P. per mouse. For immunohistochemical perfusion, the dose was 300ul per mouse. All animals were humanely killed at the end of the experiments. Following immunohistochemistry performed 3 days, 1 week and 3 weeks after axotomy, no change in the pattern or levels of OPN expression were noted. To investigate the possibility that OPN may play a role in pain processing, thermal and mechanical nociceptive testing was performed on intact OPN knockout and strain matched wild-type mice. Further, mechanical allodynia was measured after a partial sciatic nerve injury (SNI, a model of neuropathic pain behaviour(3)) in mice of both genotypes. No difference in the responses of knockout and wild-type animals were noted in the intact animals nor after SNI.