Conditioned runners may be able to endure higher levels of stress and pain during racing due to the presence of elevated endogenous opiates. To test this hypothesis, intravenous naloxone 10 mg or saline was administered in a repeated measures randomized, double‐blind protocol to 15 male runners. About 20 minutes after the injection, the subjects ran a 400 meter race, with the elapsed time being recorded. Pre‐and post‐run mood and post‐run global pain status were measured. Contrary to the hypothesis, naloxone had no statistically significant effect on mean race time, mean pre‐to post‐race mood, or upon a post‐race global pain rating when compared with saline; a trend in the analysis (p = 0.12) did suggest that naloxone injection was associated with faster run times. Three runners showed a marked improvement in racing time following naloxone, suggesting that some athletes may gain advantage by having blocked endorphins during maximal performances.