Using oligomer primers based on the cDNA sequence of human interleukin 1 beta converting enzyme (ICE), we have employed the RT-PCR method and rat spleen RNA to clone and sequence rat ICE. We report here that the predicted amino acid sequence of rat ICE proenzyme consists of 402 amino acids (p45) and shares 61% and 90% identity, respectively, with human and mouse ICE amino acid sequences. The active site cysteine (Cys284) and 3 or 3 potential processing sites are conserved suggesting that their the rat ICE heterodimer consists of a p22 (Ser104-Asp296) and a p10 (Gly315-His402) subunit or a cryptic processing site creates a smaller heterodimer. Northern blot analysis has revealed a approximately 2.2 kb and a more abundant approximately 1.45 kb ICE transcript both widely expressed in the rat with the highest expression in spleen and intestine and lowest in brain. IL-1 beta mRNA was similarly distributed. Injection of the immunostimulant, lipopolysaccharide (0.2 mg/kg, i.p.), increased rICE mRNA content between 2- to 3-fold in the rat brain with smaller increases measured in testis and spleen. The structural conservation of this enzyme suggests that rat models of inflammation will be useful for evaluating the therapeutic potential of ICE inhibitors in humans.