1. Intrinsic heart rate (IHR: heart rate following autonomic blockade with atropine and propranolol) increases with exercise. The opioid antagonist naloxone has been shown to decrease IHR at rest, raising the possibility that increases in IHR with exercise are beta-endorphin related, since beta-endorphin concentrations have also been shown to rise during exercise. 2. We examined the effects of naloxone (10 mg) on IHR and plasma beta-endorphin levels during aerobic exercise in eight healthy, male subjects in a single blind, crossover study. 3. IHR increased with 25 min bicycling from 97.1 +/- 1.4 to 129.7 +/- 1.2 beats min-1 (mean +/- s.e. mean). This rise was not affected by administration of naloxone. 4. Plasma beta-endorphin concentration rose from 31.1 +/- 3.8 to 94.9 +/- 23.9 pg ml-1 after 25 min exercise. This exercise-induced rise in beta-endorphin concentration was further increased (P less than 0.05) in the presence of naloxone. 5. Our results confirm a rise in IHR and beta-endorphin concentrations with acute exercise but indicate that the changes in IHR are not endorphin-related.