Drug disposition is of prime concern to pharmacologists, toxicologists, and clinicians. In many cases the variation in absorption, distribution, metabolism, and excretion of a series of congeners is governed by the molecular structure and the physicochemical properties of the drug molecule. When reproducible results of sufficient spread are available, these data can be analyzed in quantitative terms. In order to achieve a high degree of selective toxicity in designing better and safer therapeutic agents, the multifacet effects of molecular modification on drug disposition must be thoroughly investigated. It is hoped that more systematic effort will be directed toward the correlation of various pharmacokinetic parameters and therapeutic and toxicological effects of drugs with their physicochemical properties and molecular structures.