The aims of this study were to examine the effects of nicotine on resting energy expenditure (REE) and plasma catecholamine release following ad libitum smoking in fasted, middle-aged males; and, to determine if the acute responses in REE and catecholamine release to smoking differ between normal weight (< or = 25 kg/m(2)) and overweight (> 25 kg/m(2)) smoker groups. REE was measured by indirect calorimetry in 32 fasted male smokers prior to, and 30 minutes after, consuming either two 0.16 mg nicotine (low yield) or two 1.74 mg nicotine (high yield) cigarettes. Plasma nicotine and norepinephrine levels were simultaneously measured. There was no thermic effect of nicotine or catecholamine response after smoking low yield cigarettes in both groups, nor after smoking high yield cigarettes in the overweight group. In contrast, REE increased 7.2%, which was accompanied by an increase in plasma norepinephrine release, after smoking high yield cigarettes in the normal weight group. Controlling plasma nicotine level ablated these increases. In conclusion, body mass interacts with the thermic effect of nicotine and neuroendocrine function in male smokers. Smoking-induced increase in the plasma nicotine level accounts for the observed responses, which are blunted in overweight smokers. This finding may have implications for potential weight gain following smoking cessation by normal weight individuals, related to lost smoking-induced thermogenesis.