ABSTRACT
This chapter provides an overview of the basic factors that affect biological response to chemicals, reviews the anatomical and physiological considerations in animals species selections, and describes several examples of species variations in biological response and the reasons for those variations. Biological and biochemical factors modify toxicity and must be considered in the selection of species to be used for toxicity studies. Metabolism, kinetics, the presence of chemicals in tissues, and biotransformation must be studied increasingly for a better understanding of species differences. The variables that affect the target cell, the presence of target organelles within the cell, the ratio of strong versus weak binding sites within cells, the location of receptor sites, the presence of other chemicals as promoters, and the structure and properties of target enzymes are necessary details. This knowledge is needed and will assist in evaluating why one species is more or less sensitive to a specific chemical group.
