Abstract. Nine highly fit men [mean (SE) maximum oxygen uptake, [MATHEMATICAL FORMULA] [MATHEMATICAL FORMULA] : 63.9 (1.7)mlkg1min1; age 27.6 (1.6)years] were studied during two treadmill exercise trials to determine plasma -endorphin immunoreactivity during intense exercise (80% [MATHEMATICAL FORMULA] [MATHEMATICAL FORMULA] ). A double-blind experimental design was used, and subjects performed the two exercise trials in counterbalanced order. Exercise trials were 30min in duration and were conducted 7days apart. One exercise trial was undertaken following administration of naloxone (1.2mg; 3cm3) and the other after receiving a placebo (0.9% NaCl saline; 3cm3). Prior to each experimental trial, a flexible catheter was placed into an antecubital vein and baseline blood samples were collected. Thereafter, each subject received either a naloxone or placebo bolus injection. Blood samples were also collected after 10, 20 and 30min of continuous exercise. -Endorphin was higher (P0.05) during exercise when compared to pre-exercise in both trials. However, no statistically significant difference was found (P0.05) between exercise time points within either experimental trial. -endorphin immunoreactivity was greater (P0.05) in the naloxone than in the placebo trial during each exercise sampling time point [10min: 63.7 (3.9)pgml1 vs 78.7 (3.8)pgml1; 20min: 68.7 (4.1)pgml1 vs 83.8 (4.3)pgml1; 30min: 71.0 (4.3)pgml1 vs 82.5 (3.2)pgml1]. These data suggest that intense exercise induces significant increases in -endorphin that are maintained over time during steady-rate exercise. Exercise and naloxone had an interactive effect on -endorphin release that warrants further investigation.