Genes from all 3 organisms encode predicted 579-amino acid proteins that each contain a putative transmembrane domain, an amidase consensus sequence, and a putative SH3-binding motif. The sequence of the human protein is 82% and 84% identical to those of rat and mouse FAAH, respectively. Mammalian cells expressing rat or human FAAH exhibited high levels of oleamide hydrolase activity. Western blotting and cell fractionation studies indicated that this activity was associated with a 60 to 65 kD membrane protein. Human and rat FAAHs hydrolyzed a similar variety of fatty acid amides, although the human enzyme was quantitatively somewhat less selective. Northern blot analysis revealed that human FAAH is expressed as a 2.1-kb mRNA in various tissues, but not in heart or lung.