In studies on nitrosating agent(s) formed in skin of mice exposed to nitrogen dioxide, we showed that: (i) N-nitrosomethylaniline was produced in skin of mice exposed to nitrogen dioxide and then painted with N-methylaniline; (ii) a nitrosating precursor in methyl linoleate is associated with peroxidation products; (iii) cholesterol is a major nitrosating precursor in mouse skin, probably because it produces the nitrosating agent, cholesteryl nitrite; (iv) cholesteryl nitrite enhances autoxidation of lipids in vivo and on mouse skin and, like sodium nitrite, catalyses the autoxidation of iodide; (v) N-nitrosomethylaniline was produced in mice injected intraperitoneally with methylaniline and gavaged with amyl nitrite; and (vi) nitrosating agents may occur normally in human skin lipids.