Latencies and phosphomannosyl-enzyme receptors of lysosomal enzymes were studied in the skeletal muscles of NMRI mice during the appearance (0-1 days) and the repair (3-9 days) of muscle fiber injuries after a single bout of prolonged running (9 hr, 13.5 m/min). The unsedimentable, releasable, and bound activities of arylsulfatase, beta-N-acetylglucosaminidase, beta-D-glucuronidase, cathepsin C, and ribonuclease as well as the content and occupancy of phosphomannosyl-enzyme receptors of lysosomal enzymes were assayed. The distribution of enzyme activities in different fractions as well as the changes after exertion greatly varied between different lysosomal enzymes. In general, the total activities and also the distribution of enzyme activities in different fractions were unaffected 1 hr after exertion, but on the day after exertion small increases were observed in the free and releasable activities. The highest enzyme activities both in the homogenate and in different fractions were recorded 3 days after exertion, after which the activities slowly decreased. The increases of enzyme activities were higher in the free and releasable fractions than in the homogenate but the changes in the proportional distributions of lysosomal enzyme activities between different fractions were minor. The present study also showed the presence of phosphomannosyl-enzyme receptors of lysosomal enzymes in the membranes of skeletal muscles. The total content of phosphomannosyl-enzyme receptors was unchanged 0-3 days after exertion but a small increase occurred 5-8 days after exertion. Instead, the occupancy of these lysosomal receptors with endogenous enzymes was significantly increased 1-5 days after exertion and decreased later to the control level.