A 28-day study was conducted with 13 adult men to determine the effect of weight lifting exercise and protein intake level on urinary 3-methylhistidine (3MH) excretion. Subjects were fed the RDA for protein [0.8 g/(kg BW X d)] or 3 X RDA; there were no-exercise and exercise groups at each intake. Comparisons of last 14-day, least-squares means among groups did not reveal differences in data treated by lean body weight [3MH/(kg LBW X d)] or by urinary creatinine excretion [3MH/(kg UCE X d)], but 3MH/(kg LBW X d) excretions were higher for exercise than no-exercise subjects. Regression analyses revealed linear, increasing trends in the 3MH/(kg LBW X d) data for RDA-exercise (p less than 0.03), 3 X RDA-exercise (p less than 0.01), and 3 X RDA-no-exercise (p less than 0.01) groups; 3MH/(g UCE X d) group data plots overlapped. Our findings for 3MH/(kg LBW X d) indicate that a weight lifting program was associated with increased 3MH excretions from adult males. As an index of skeletal muscle catabolism, an increase in 3HM excretion represents an increase in tissue catabolism. No significant effect of 3 X RDA protein intake on last 14-day 3MH excretions was observed; however, linear increases in 3MH/(kg LBW X d) for 3 X RDA-no-exercise subjects suggests a relationship. Trends of exercise- or protein intake-enhanced 3MH excretion could be masked by data as 3MH/(g UCE X d) if exercise or 3 X RDA protein intake can expand the body creatine pool independent of skeletal muscle mass.