The effect of an acute bout of high-intensity concentric exercise on serum muscle and collagen marker proteins was studied in nine male students. The muscle-derived serum carbonic anhydrase III, myoglobin, and creatine kinase all increased as a result of the exercise. Serum type I procollagen carboxyterminal propeptide decreased at first but started to increase 2 days after the exercise. Serum galactosylhydroxylysyl glucosyltransferase was elevated immediately after the exercise. No significant changes were seen in the concentrations of serum amino-terminal propeptide of type III procollagen or 4-hydroxyproline. It seems that a single bout of heavy concentric exercise causes protein leakage from muscles and probably from the collagen-synthesizing cells of the connective tissue, which may be accompanied by an initial decrease and a subsequent increase in type I collagen production. The activation of type I collagen production seems to depend on the strain and damage of the musculoskeletal system.