In order to gain information about the effect triggered at the molecular level by nicotine, its neuroimmunomodulatory properties and its impact on the pathogenesis of inflammatory diseases, peripheral blood serum and leukocytes of rat submitted to passive nicotine administration were subjected to proteomic investigation. Serum, polymorphonuclear (PMN) and mononuclear (MN) leukocytes from chronically treated animals and from control animals were analysed by a two-dimensional (2-D) gel electrophoresis/mass spectrometry approach to detect differentially expressed proteins. The nicotine regimen selected is known to have a stimulatory effect on locomotor activity and to produce a sensitisation of the mesolimbic dopamine system mechanism involved in addiction development. After 2-D gel analysis and matching, 36 spots in serum, seven in PMN and five in MN were found to display a statistical difference in their expression and were subjected to matrix-assisted laser desorption/ionization-time of flight-mass spectrometry peptide fingerprinting for protein identification. Fifteen different proteins were identified. The results indicate an overall impact of nicotine on proteins involved in a variety of cellular and metabolic pathways, including acute phase response (suggesting the effect on inflammatory cascades and more in general on the immune system), oxidative stress metabolism and assembly and regulation of cytoskeleton. In particular, the observed changes imply a general reduction in the inflammatory response with a concomitant increased unbalance of the oxidative stress metabolism in the periphery and point to a number of potential noninvasive markers for the central nervous system (CNS) and non-CNS mediated activities of nicotine.