Interleukin (IL)-1 is a major contributor to inflammation and apoptosis during ischemia/reperfusion (I/R) injury. Its deleterious effects are primarily mediated by the activation of nuclear factor-kappaB (NF-kappaB). Receptor-binding and signaling of IL-1 can be blocked by the IL-1 receptor antagonist (IL-1ra). The aim of our study was to characterize effects and mechanisms of IL-1ra administration on inflammation, apoptosis, and infiltration in renal I/R injury. Renal ischemia was induced in Lewis rats by clamping of the left renal artery for 45 min. Kidneys were removed for histological and molecular analysis 24 h or 5 days after reperfusion. IL-1ra ameliorated I/R induced renal injury and inflammation. Furthermore, the number of apoptotic tubular cells was lower in IL-1ra-treated animals 24 h after ischemia, which was paralleled by a Bax/Bcl-2 mRNA ratio towards anti-apoptotic effects. IL-1ra reduced the expression of monocyte chemoattractant protein-1 (MCP-1) mRNA at 24 h and 5 days and that of intracellular adhesion molecule-1 (ICAM-1) expression at 24 h in the ischemic reperfused kidneys. Our results indicate that IL-1ra treatment ameliorates renal I/R injury and this protective effect might be mediated by reduced induction of NF-kappaB mediated MCP-1, ICAM-1, and a decreased ratio between Bax and Bcl-2 mRNA expression.