Department of Health and Sport Science, Robins Center, University of Richmond, Virginia 23173.

Previous research investigating the response of plasma beta-endorphins (beta-EP) to resistance exercise has resulted in equivocal findings. To further examine the effects of resistance exercise on beta-endorphin immunoreactivity, six resistance-trained athletes participated in a three-set series of eight repetitions of isotonic exercise. All exercises were performed at 80% maximal effort. Blood was sampled from the group by venepuncture, both prior to and following the exercise bout, and beta-endorphin concentration was determined by radioimmunoassay. The results indicated that mean (+/- S.E.) plasma levels of beta-endorphins following exercise (18.04 +/- 3.4 pg beta-EP ml-1) were not significantly changed from pre-exercise (control) levels (19.59 +/- 2.4 pg beta-EP ml-1), although there was considerable inter-individual variability. Our results support previous research which has reported no significant changes in beta-endorphin immunoreactivity following resistance exercise, as well as reported findings of considerable variability in the beta-endorphin response to exercise.