Kinetics are involved in virtually all environmental processes. However, only those rate processes which are within a few orders of magnitude of the examined biological rates are of practical importance. One of the most important determinants is the influence of body size on the different metabolic rates at which organisms live. Rather than continuing to examine fundamental influencing factors, the authors propose to discuss kinetics within a toxicological framework, and on a process-orientated basis within that framework. Kinetics come into play in all three of the basic areas defined in classic toxicology/pharmacology: Exposure--those factors external to the organism that control contact with the chemical; Toxicokinetics--those physiological and behavioral factors controlling uptake, distribution, metabolic processing, elimination and, ultimately, delivery to the site(s) of toxic action.