Xenobiotics are molecules that are introduced into the body from the environment and subsequently metabolized by the body. Most pharmacological agents are lipophilic and their elimination from the body is facilitated by enzymatic modifications to render them more hydrophilic. The liver is a major site for xenobiotic metabolism. In addition to foreign substances, a number of endogenous substances, including some eicosanoids and steroid hormones, are also modified to facilitate their excretion.