PIVARNIK, J. M., J. F. HICKSON, JR., and I. WOLINSKY. Urinary 3-methylhistidine excretion increases with repeated weight training exercise.Med. Sci. Sports Exerc., Vol. 21, No. 3, pp. 283–287, 1989. This investigation examines the effect of progressive resistance weight training exercise on urinary 3-methylhistidine (3-MH) excretions in untrained subjects. For 19 consecutive days, 11 males were fed a weight maintenance, lactovegetarian diet which contained the Recommended Dietary Allowance (0.8 g·kg-1·d-1) for protein. No exercise was performed for the first 7 d of the study. Subjects were strength tested on day 8 and performed upper and lower body weight training exercises from days 9–19. Complete, 24-h urine collections were obtained from each subject on a daily basis. Samples were assayed for creatinine and 3-MH. Stable baseline 3-MH values were present during the pre-exercise control period. Significant increases in 3-MH occurred by study day 11, which was the third day of weight training exercise. This was true regardless of whether the data were expressed by daily excretions (μM·d-1;P< 0.01), per unit of body weight (μM· kg-1d-1;P< 0.01), or per unit of creatinine excretion (μM·g Creat-1·d-1;P< 0.001). Since urinary 3-MH is an index of actin and myosin catabolism, these data support the hypothesis that the rate of skeletal muscle degradation is increased during strength building exercises.