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LOBSTEIN, D. D. and A. H. ISMAIL. Decreases in resting plasma beta-endorphin/-lipotropin after endurance training. Med. Sci. Sports Exerc, Vol. 21, No. 2, pp. 161–166,1989. The purpose of the present study was to investigate changes in the resting plasma beta-endorphin/ -lipotropin (βE/β-LPH) concentration that may occur with exercise training. A treatment group (N=10) of previously sedentary, psychologically normal, and medically healthy middle-aged men trained for 4 months in a structured fitness program. The treatment group was compared with two age-matched groups: a jogger group (N=10) and a sedentary group (N=10). Venous blood was collected in the early morning after the subjects had been fasting for at least 12 h. The subjects were also resting at the time of blood collection. Resting plasma βE/β-LPH (measured by radioimmunoassay) decreased (P<0.05) following the 4 months of exercise training in the treatment group, from 11.01 ± 1.62 (SE) to 7.22 ± 0.99 pmol·l-1. The jogger and sedentary groups demonstrated no changes in the resting plasma βE/β-LPH concentration following the 4 months. Aerobic fitness, measured by the maximal oxygen uptake (O2max), increased (P<0.01) from 36.56 ± 2.58 (SE) to 44.85 ± 1.34 ml·kg-1·min-1in the treatment group after the 4 months of training. There were no changes in O2maxfor either the jogger group or the sedentary group following the 4 months. In conclusion, βE/β-LPH in the plasma of resting middle-aged men appeared to be decreased after 4 months of aerobic training.