This metabolism of administered chemical should be relevant to human or domestic animals and is critical for risk assesent exercises. The final toxic effect will depend on a balance between the level of toxic agent reaching the target tissue and its rate of elimination and/or bioinactivation by mixed function oxidases, serhyfrolases or binding to serum proteins. Furthermore, there are interspecies differences regarding the metabolism of xenobiotics (Nebbia, 2001). For example, cats are at high risk of developing hepatotoxicity specially after paracetamol administration. This is due the differences inbooactivation of paracetamol, which occurs only in cats through N-hydroxylation with the help of cytochrome P450 2E1, during the oxidative reactions in phase I transformation (Nebbia, 2001). Ruminants are less susceptible to organophosphates (OPs) such as parathion than monogastrics, because the rumen microflora plays an important role by reducing the nitro group of OPs to an amino group (Nebbia, 2001).