In brief: Serum beta-endorphin/beta-lipotropin levels were measured in six elite kayak paddlers and 14 untrained subjects before and after an isometric knee-extension exercise regimen designed to induce profound muscle exertion and discomfort. We found no significant difference in resting beta-endorphin/beta-lipotropin levels between the two groups. Beta-endorphin/beta-lipotropin levels were not altered by the isometric exercise in either group despite a strength loss of approximately 34% and severe muscle exertion and discomfort. These data suggest that previously reported increases in beta-endorphin levels in response to other exercise regimens may not function primarily to modulate the exercise-induced muscle discomfort and pain.