In evoking pharmacological and toxicological responses, drugs generally combine either reversibly or irreversibly with action sites on intrastitial macromolecules or organelles, and thereby cause alterations of physico-chemical or biochemical processes in the living animal. These alterations can be evoked by a number of mechanisms. Drugs can either mimic or block the action of normally occurring substances by combining with receptor sites. Drugs can alter the localization of normally occurring substances by interfering with transport mechanisms or changing the number of storage sites. Drugs can change the concentrations of normally occurring substances in tissues by reacting with them directly or by altering the activities of enzymes that catalyze their formation or destruction.