The effects of nicotine administration (2 mg eight-times daily as nicotine chewing gum for two weeks) on plasma lipid and lipoprotein concentrations were studied in young healthy volunteers. Plasma levels of the nicotine metabolite, cotinine, reached levels comparable to those seen in smokers. Plasma concentrations of triglyceride, cholesterol, HDL cholesterol, LDL cholesterol, and apolipoproteins AI and B, were determined repeatedly before, during and after cessation of nicotine intake. All these variables, as well as the activities of lipoprotein lipase and hepatic lipase in post-heparin plasma, remained unchanged throughout the study. The results strongly suggest that the effects of smoking on plasma lipoprotein metabolism are not mediated via nicotine, and indicate that nicotine chewing gum, when used therapeutically in anti-smoking programmes, does not carry the same metabolic side effects as smoking.