OBJECTIVE: The effects of acute nicotine administration and smoking on brain function were investigated in two studies, with the primary goal of identifying neural systems that mediate these effects. METHOD: In study 1, 18 healthy volunteer cigarette smokers were exposed to three conditions in a single session: 1) smoking a nicotine-containing cigarette, 2) smoking a denicotinized cigarette, or 3) receiving intravenous nicotine injections in conjunction with smoking a denicotinized cigarette. In study 2, 16 subjects smoked a nicotine-containing and denicotinized cigarette in each of two sessions 2 hours after receiving the nicotinic antagonist mecamylamine (10 mg) or placebo orally. Regional cerebral blood flow (rCBF) was assessed by using the bolus (15)O-labeled water method and positron emission tomography. Subjective measures of smoking withdrawal symptoms were also collected. RESULTS: A principal-components analysis of rCBF data pooled from the two studies identified three factors consisting of frontal, striatal, and reticular systems. The amygdala was considered as a separate region of interest. Nicotine increased normalized rCBF in the left frontal region and decreased rCBF in the left amygdala. The rCBF in the right hemisphere reticular system was related to nicotine dose in an inverted-U-shaped pattern and was strongly related to self-reported craving for cigarettes and to the addiction scale of a smoking motivation questionnaire. The effects of mecamylamine on rCBF were generally opposite to those of nicotine. CONCLUSIONS: The results indicate that nicotine influences brain regions involved in arousal and reward and suggest specific functional systems that may be linked to motivationally significant aspects of tobacco dependence.