KRAEMER, W. J., S. J. FLECK, R. CALLISTER, M. SHEALY, G. A. DUDLEY, C. M. MARESH, L. MARCHITELLI, C. CRUTHIRDS, T. MURRAY, and J. E. FALKEL. Training responses of plasma beta-endorphin, adrenocorticotropin, and cortisol.Med. Sci. Sports Exerc, Vol. 21, No. 2, pp. 146–153, 1989. The purpose of this study was to examine the effects of three different run training programs on plasma responses of beta-endorphin (β-EP), adrenocorticotropin (ACTH), and cortisol to maximal treadmill exercise. Subjects were randomly assigned to one of three training groups: sprint intervals (SI) (N= 8), endurance (E) (N= 10), or combination (C) (N= 7). Training was monitored for 10 wk, and maximal treadmill exercise tests were administered pre-training and after 2, 4, 6, 8, and 10 wk of training. Blood samples were obtained (pre-training and after 10 wk) before, immediately after, and 5 and 15 min following the maximal exercise tests. All groups significantly (P< 0.05) increased maximal oxygen consumption values at 8 and 10 wk of the training period. Significant exercise-induced increase in plasma β-EP, ACTH, cortisol, and blood lactate were observed for both pre- and post-training tests in all training groups. The SI group demonstrated significant post-training increases in β-EP, ACTH, cortisol, and 5 min post-exercise blood lactate concentrations in response to maximal exercise. No training-induced hormonal changes were observed for the E group. While exercise-induced increases were observed, the C group exhibited significant post-training reductions in plasma responses ofβ-EP, ACTH, and blood lactate concentrations in response to maximal exercise. Still, resting and post-exercise increases in plasma cortisol concentrations were significantly higher in magnitude in the post-training test. Lactate was significantly correlated with β-EP (r = 0.72), ACTH (r = 0.70), and cortisol (r = 0.64). These data suggest that different run training programs produce differential effects on plasma β-EP, ACTH, and cortisol in response to maximal exercise and that these responses may be linked to anaerobic metabolic factors.