MCMURRAY, R. G., W. A. FORSYTHE, M. H. MAR, and C. J. HARDY. Exercise intensity-related responses of β-endorphin and catecholamines.Med. Sci. Sports Exerc., Vol. 19, No. 6, pp. 570–574, 1987. Ten men and 10 women exercised on a bicycle ergometer for 20 min at 40, 60, and 80% maximal oxygen uptake (VO2max) to determine the relationship between plasma β-endorphin, catecholamines, and exercise intensity. Compared to rest, plasma β-endorphins were not significantly elevated during the 40 and 60% workloads (4.8 ± 1.0 pmol.1-1vs 3.8 ± 0.7 and 6.3 ± 0.9, respectively). In contrast, the 80% exercise significantly elevated endorphins to 16.1 ± 4.0 pmol.1-1. Plasma norepinephrine concentrations were 0.30 ± 0.04 ng.ml-1at rest and increased with exercise intensity (40% = 0.60 ± 0.05, 60% = 0.93 ± 0.07, 80% = 2.00 ± 0.14, VO2max= 2.55 ± 0.14 ng.ml-1). Plasma epinephrine followed the same trend (rest = 0.07 ± 0.01, 40% = 0.33 ± 0.03, 60% = 0.49 ± 0.02, 80% = 0.88 ± 0.07, VO2max= 0.95 ± O.06 ng.ml-1). Norepinephrine was found to significantly correlate to endorphins (r = 0.499;P< 0.02). Conversely, epinephrine was not correlated with β-endorphin (r = 0.309;P> 0.05). The low correlation suggests a weak relationship between β-endorphin and catecholamine responses during exercise. The results of this investigation suggest that the relationship between β-endorphin and exercise intensity is curvilinear, with anaerobic activity producing the most significant endorphin response. It was also noted that the β-endorphin response was not related to gender, but the amine response to exercise was gender-related, being greater for the men.