The present study explored neuropeptide responses to nicotine from smoking. Habitual smokers smoked research cigarettes of known strength under controlled laboratory conditions while blood samples were withdrawn unobtrusively for subsequent biochemical analysis. To provide a metric that reflected total nicotine intake and total neurohormonal output, data were integrated over time. Subjects were relatively unresponsive in the low-nicotine (0.48 mg) condition. In the high-nicotine (2.87 mg) condition, there were significant positive correlations between integrated plasma nicotine and plasma arginine vasopressin (r = +0.985), its carrier protein neurophysin I (r = +0.944), and beta-endorphin-beta-lipotropin (r = +0.977), but not adrenocorticotropic hormone. Data from an experiment that used an extraction step to remove beta-lipotropin corroborated the functional relationship between plasma nicotine and beta-endorphin implied by the original findings. Taking into account recent research on the role of neuropeptides in the modulation of affective states and cognitive function as well as of other CNS activity, the present findings were interpreted as strengthening the hypothesis that nicotine-stimulated neuropeptide release provides positive reinforcement for smoking.