It is well known that repeated injections of nicotine produce progressively larger increases in locomotor activity, an effect referred to as behavioral sensitization. This study was carried out to investigate the neural mechanisms underlying nicotine-induced behavioral sensitization using in vivo microdialysis and Fos-like immunohistochemistry (FLI). Rats were given repeated injections of saline or nicotine (0.4 mg/kg s.c., twice daily for 7 days) followed by one challenge injection on the 4th day after the last daily injection. Systemic challenge with nicotine produced a much larger increase in locomotor activity in nicotine-pretreated rats (659.1+/-94.9 counts/2 h) than in saline-pretreated rats (218.1+/-61 counts/2 h). A direct local challenge of nicotine (1 or 5 mM) via a microdialysis probe in the nucleus accumbens or striatum induced a much greater dose-dependent increase of dopamine (DA) output in nicotine-pretreated rats than in saline-pretreated rats. Furthermore, in parallel with the behavioral and biochemical data, systemic challenge with nicotine produced marked Fos-like immunohistochemistry in the nucleus accumbens and the striatum in the nicotine-pretreated rats. Taken together, this study demonstrates that behavioral sensitization is clearly associated with an increase in DA release and activation of Fos-like immunoreactive cells in the striatum and the nucleus accumbens produced by repeated nicotine treatment. Our results strongly suggest that the striatum and the nucleus accumbens may play a major role in nicotine-induced behavioral sensitization. The present results are discussed in terms of the development and expression of nicotine-induced behavioral sensitization.