The activities of prolyl 4-hydroxylase and beta-glucuronidase, the concentration of hydroxyproline as well as reticulin and collagen type III, IV and V stainings were followed in skeletal muscle during a 20-day period after a 9-h treadmill running in untrained and trained male mice, aged 4-6 months. The prolonged 9-h running of untrained mice temporarily increased prolyl 4-hydroxylase activity 2, 5 and 10 days after exercise, more prominently in the red than in the white part of quadriceps femoris-muscle, and in analogical manner as beta-glucuronidase activity in tibialis anterior-muscle. Twenty days after exercise these enzymatic activities were back to the control level. The hydroxyproline content of red muscle was increased for 10 and that of white muscle for 20 days after the exertion. Training for 45 days did not affect hydroxyproline content and prolyl 4-hydroxylase activity was at the control level after the training. A 9-h exercise increased prolyl 4-hydroxylase activity much less in trained muscle than in the untrained muscle and did not affect muscle collagen content. Histological observations showed fiber necrosis 2 days and signs of fiber regeneration 5 days after the exertion in untrained mice. Twenty days afterwards the regeneration was nearly completed. Reticulin staining was increased in injured muscle areas 10-20 days after the exertion. In immunohistochemical staining, antibodies to all studied collagen types (type III, IV and V) showed increased staining 5-20 days after the exertion in the areas of muscle injuries and regeneration.(ABSTRACT TRUNCATED AT 250 WORDS)