Although various kinds of environmental factors may alter the activity of cytochrome P-450 enzymes in liver micromes, their effects on the pharmacokinetics of drugs and other foreign compounds in living animals may not be as great as might be predicted from assays of these enzymes in vitro. Indeed, the effects will depend on the relative importance of excretory and metabolic mechanisms in the elimination of the drug, the relative importance of various metabolic reactions in different tissues, the extraction ratio of the drug by the liver, and in some instances on the route of administration of the drug. Moreover, the effect of the various environmental factors on the pharmacologic and the toxicologic actions of the drug will depend on whether these actions are caused by the parent foreign compounds or by one or more of their metabolites. It may also be important that the environmental factors may alter not only relative activiteis of the cytochrome P-450 in liver microsomes but also the activities of other drug-metabolizing enzymes and that the relative effects of the environmental factors of these enzymes may differ depending on the animal species or the animal strain. Indeed, a given factor may increase the pharmacologic effects of a drug metabolite in one animal species but decrease it in another. For these reasons, it frequently is not possible to predict the effects of environmental factors on drug action in living animals solely from in vitro rates of metabolism of model substrates.