Acute mood changes occur with various forms of physical activity. Increased levels of endogenous opioids (endorphins) in response to exercise may mediate activity-induced shifts in mood state. Thirteen female and six male aerobics class participants aged 20-46 years received the opiate receptor antagonist naltrexone and a placebo in randomized, double-blind crossover fashion on two separate occasions at the same 75-min high-intensity aerobics class. Mood states were assessed before and after each class, which were spaced 5 days apart, using the Profile of Mood States questionnaire (POMS), a mood adjective checklist, and a Visual Analogue Scale (VAS) which measured mood in relation to several emotional extremes. Mood changes over the course of each aerobics class were compared in the naltrexone and placebo groups. For men and women, significant differences between conditions were observed in overall mood by both the POMS (P < 0.005) and VAS (P < 0.02). There were significant differences between conditions for most subscales of each mood instrument (P < 0.05); with the placebo, mood states became calmer, more relaxed and pleasant, tending away from depression, anger and confusion. Positive mood shifts did not occur when subjects were preloaded with naltrexone, suggesting that activity-generated mood changes are mediated through endorphinergic mechanisms.