This within-subjects, placebo-controlled laboratory study was designed to examine the effects of naltrexone on cigarette response in 44 chronic smokers (23 male, 21 female). Each participant received either 50-mg oral naltrexone or identical placebo during the morning of the session after maintaining overnight abstinence. Subsequently, the participant was administered a smoking cue (holding lit cigarette) to examine craving and associated features of smoking, and instructed to smoke a cigarette 1 h later. This was followed by a smoking interval in which participants could choose to smoke up to four more cigarettes over a 2-h period. Subjective measures (withdrawal, craving, affect, and side effects) and smoking behavior were assessed throughout the session. Naltrexone significantly reduced the total number of choice cigarettes smoked and expired carbon monoxide levels (Ps<.05). Naltrexone significantly increased total side effects, especially for sedation (P<.01). Further, naltrexone significantly increased overall negative affect, and decreased positive affect 1 h after smoking the first cigarette (Ps<.05). However, naltrexone did not affect acute withdrawal or smoking urges. Despite mixed findings, women reported more overall naltrexone-induced withdrawal (P<.05) and side effects (P<.08) compared to men. Although the exact mechanism is unknown, the findings support an opioid antagonist attenuation of smoking behavior.