Cytochrome P450s (P450 or CYPs) comprise a superfamily of enzymes that catalyze the oxidation of a wide variety of xenobiotic chemicals including drugs and environmental carcinogens. Recent studies have demonstrated that endogenous chemicals are also oxidized by human P450s which mainly metabolize xenobiotics. In this review, we summarize the expected physiological significance of the biotransfornation as well as Michaelis-Menten constants (Km), maximal velocities (Vmax), Vmax/Km (intrinsic clearance) values, and/or metabolic activities for 33 endogenous substrates, including (1) arachidonic acid and fatty acids, (2) steroid hormones, such as testosterone, progesterone, and allopregnanolone, (3) amines, such as tyramine, and (4) lipid-soluble vitamins, such as retinol and vitamin D3 analogues, mediated human P450 isoforms consisting of so-called drug-metabolizing enzymes for the purpose of predicting the key enzyme(s) in vivo. Arachidonic acid is metabolized via the epoxidation and ωhydroxylation to many biologically active eicosanoids such as epoxyeicosatrienoic acids and hydroxyeicosatetraenoic acids by multiple P450 isoforms including CYP2C, CYP2E1 and CYP4A11. CYP2D in the brain may be involved in the metabolism of neuronal amines and steroids and in the regulation of the central nervous system. CYP1A2 and CYP3A4 appear to be the major P450 enzymes catalyzing the oxidation of all-trans-retinol to all-trans-retinoic acid in human liver, and CYP3A4 is one of the vitamin D3 25-hydroxylases. Although the significance of the contribution is still unknown in detail, the collective findings provide fundamental and useful information for the biological contribution of the metabolism of endogenous substances by drug-metabolizing enzymes, P450s. In addition, genetic polymorphism of these drug-metabolizing P450s may affect the metabolism of the endobiotics. Forthermore, these findings imply that xenobiotic oxidations by P450 enzymes are affected by endobiotic molecules and that the endobiotic-xenobiotic interactions as well as drug-drug interactions or drug-food/beverage interactions may be of great importance when understanding the basis for pharmacological and toxicological actions of a number of xenobiotic chemicals.