Cigarette smoking is associated with increased incidence of periodontal disease and poor response to periodontal therapy. Several studies have shown the detrimental effects of nicotine on periodontal tissue. To investigate the molecular toxicological implications of cigarette smoking on periodontal tissue, expression of c-fos early stress response gene was examined in human periodontal ligament fibroblasts (PDLFs) after exposure to nicotine. The exposure of quiescent human PDLFs to nicotine resulted in the induction of c-fos mRNA expression. The levels of the c-fos mRNAs increased about 2.5 and 4.8-fold after exposure to 2.5 mm and 10 mm nicotine for 2 h, respectively. Moreover, the peak of c-fos mRNA levels induced by nicotine was 5 mm at 2-h incubation period. Kinetic investigations of c-fos mRNA expression in nicotine-treated cells revealed a rapid accumulation of the transcript, a significant signal first detectable after 30 min of exposure. This increase was transient and the level of c-fos mRNAs returned rapidly to that of control cells by 8 h. To determine whether thiol levels were important in induction of c-fos by nicotine, we pretreated cells with the glutathione (GSH) precursor, 2-oxothiazolidine-4-carboxylic acid (OTZ), to boost thiol levels, or buthionine sulfoximine (BSO) to deplete GSH. Our results demonstrate that OTZ pretreatment decreased in c-fos mRNA level and BSO pretreatment enhanced in c-fos mRNA level after exposure to nicotine. In addition, nicotine significantly depleted intracellular GSH in a dose-dependent manner (P < 0.05). At a concentration of 5 mm and 20 mm, nicotine depleted about 22.2% and 56% of GSH, respectively. Taken together, c-fos gene expression might be one signal transduction pathway linked to the induction of early response genes by cigarette smoking. These results suggest that the nicotine-dependent stress-specific expression of the c-fos gene correlates with cellular thiol levels in human PDLFs.