Changes of sterol metabolism in mouse skin treated with a chemical carcinogen were studied by thin layer and gas-liquid chromatographic analyses, and it was found that some sterols disappeared from the skin. These sterols were identified as members of one of the cholesterol biosynthetic pathways, which proceeds through intermediates with saturated side chains. Furthermore, based on the results of an experiment performed with animals that had been treated with a metabolic inhibitor, azacholesterol, it is suggested that the disappearance of these sterols may be due to enzyme repression instead of increased activity of the pathway. The other biosynthetic pathway for cholesterol synthesis, which proceeds through intermediates with unsaturated side chains, was rather stimulated by the application of this chemical carcinogen to mouse skin. This change in sterol metabolism may be intimately related to the process of chemical carcinogenesis of mouse skin.