Gender effect on beta-endorphin response to exercise.Med. Sci. Sports Exerc.,Vol. 30, No. 12, pp. 1672-1676, 1998.Purpose:Twelve healthy men (26.4 yr) and women (26.8 yr) were compared at rest and after cycling for 25 min at 60 and 80% V˙O2maxto determine whether gender and menstrual cycle influenced circulating beta-endorphin concentration (BE).Methods:V˙O2maxwas determined on a cycle ergometer, and subjects completed the exercise in a randomized order. Women were tested in both the luteal (L) and follicular (F) phases of their menstrual cycle, which was confirmed by their blood estrogen levels. All tests were conducted in the morning after a 30-min rest (12-h postabsorptive). An indwelling venous catheter placed in a forearm vein enabled blood sampling at rest, 25 min of cycling, and 25 min of recovery.Results:Resting BE was similar for men before both 60 and 80% intensities of exercise, 5.27 ± 0.43 and 5.30 ± 0.33 pmol·mL−1, respectively. BE was not significantly changed at 60% V˙O2max(6.54 ± 0.33 pmol·mL−1) but significantly increased at 80% V˙O2max(11.90 ± 1.98 pmol·mL−1). Women tended to have slightly lower BE during the L compared with F, but this did not reach significance (L = 4.40 ± 0.22, F = 4.73 ± 0.30 pmol·mL−1). Cycling at 60% V˙O2maxdid not significantly increase BE in the L (5.41 ± 0.42 pmol·mL−1) nor the F (5.35 ± 0.40 pmol·mL−1). Cycling at 80% V˙O2maxincreased BE to a similar extent in both the L and F phase, respectively (10.44 and 10.96). Although the BE concentrations tended to be slightly lower in women compared with men at 80% V˙O2max, this did not reach statistical significance.Conclusions:These data suggest that women cycling at 80% V˙O2maxwill have a similar BE response to men independent of their menstrual cycle. BE in women at rest and who exercise at lower exercise intensities may have slightly lower BE levels then men independent of the time of the women's menstrual cycle.