The rewarding effects of nicotine alone and nicotine challenged with mecamylamine, a nicotine receptor blocker, or naloxone were determined using a rate-independent discrete-trial threshold measure of brain-stimulation reward in rats. If nicotine acts as other drugs of abuse, it would be expected to lower the reward threshold, that is, increase an animal's sensitivity to rewarding brain stimulation, and naloxone would be expected to block this effect, as it does other stimulants in this paradigm. Nicotine was found to significantly lower the reward threshold and mecamylamine blocked this effect. However, although naloxone increased the variability of nicotine's effect on the reward threshold, it failed to dose dependently block nicotine's threshold-lowering effect.