Xenobiotic transporters are responsible for the uptake of some chemicals into cells, and extremely important for the export of chemicals out of cells. A number of families of xenobiotic transporters have been cloned the last few years. Some microsomal enzyme inducers will enhance the plasma disappearance and biliary excretion of some xenobiotics that are not biotransformed in the intact animal, as well as in isolated hepatocytes. This is due to an up-regulation of xenobiotic transporters. As a result, some microsomal enzyme inducers will enhance the elimination and decrease the toxicity of some chemicals by enhanced transport.