The interleukin-1 (IL-1) family comprises IL-1 alpha and IL-1 beta and an endogenous IL-1 receptor antagonist (IL-1ra). IL-1 has diverse actions in the brain and has been implicated in both acute and chronic neurodegeneration. However, neither IL-1 alpha nor IL-1 beta are neurotoxic per se in vivo, so other IL-1 related ligands may be important in neurodegeneration. The cytokine interleukin-18 (also called interferon gamma inducing factor, IGIF) was first isolated from the liver of mice during toxic shock. It was later proposed as a member of the IL-1 family, based on protein sequence homology with IL-1 beta and IL-1ra, and has tentatively been called IL-1 gamma. We cloned IL-18 from adult rat brain and demonstrated, by RT-PCR, that it is expressed constitutively in cerebellum, hippocampus, hypothalamus, cortex and striatum. Rat brain IL-18 shows close homology to mouse and human IL-18, and to the recently published sequence from the rat adrenal gland. Mouse pro-IL-18 and pro-IL-1 beta are processed by caspase-1. We demonstrate that caspase-1 also cleaves rat IL-18 in vitro and that the caspase inhibitor, zVAD-DCB inhibits this cleavage.