The aims of this study were to assess the effects of a mu-opioid antagonist, naloxone, on endogenous opioid systems and to evaluate the effect of naloxone on the CNS response to mild noxious heat. Doubled-blinded experiments were performed in a cross-over design in 10 healthy male volunteers. Functional magnetic resonance imaging (fMRI) data were collected before and during the infusion and also during thermal stimuli. Increased signal was observed in a number of cortical and subcortical brain regions for naloxone versus saline infusion. Cortical activation was induced in regions including cingulate, prefrontal cortex, and insula. Subcortical regions showing increased signal change included hippocampus and entorhinal cortex. A 46 degrees C stimulus delivered to the back of the hand induced an overall increase in activation in a number of regions in the naloxone group that were not seen in the saline group (e.g., insula, orbitofrontal cortex, thalamus, and hippocampus). These results show that naloxone, even in the absence of psychophysical effects, produces activation in several brain regions that are known to have high levels of mu-opioid receptors and may be involved in endogenous analgesia. Our study is an example of how fMRI can measure subtle changes in brain activation induced by pharmacological agents without cognitive effects.