During the last decade numerous biochemical and ultrastructural studies have greatly expanded our understanding of the mechanisms involved in drug metabolism. Comprehensive recent reviews of this subject include those of Conney (1), Remmer (2), and Gillette (3). Most exogenous substances—such as pharmacologic agents, carcinogens, and insecticides—are detoxified by enzyme systems located in the microsomal fraction of the liver cell, which corresponds to the membranes of the endoplasmic reticulum seen with the electron microscope. Detoxification generally involves conversion of the molecule to a more polar form, thus rendering lipid soluble materials more water soluble for urinary excretion. This is usually accomplished