To quantify the extent of muscle alteration during prolonged exercise, the release rate of creatine kinase (CK) from striated muscle was measured in six horses during a rest period (6 h) and during three exercise tests (15, 30, and 60 km) at a constant speed of 200 m/min. CK clearance was measured after intravenous bolus administration (150 U/kg) of a CK solution obtained from horse muscle. The CK steady-state volume of distribution was 0.059 +/- 0.0215 l/kg, the terminal half-life was 123 +/- 28 min, and the plasma clearance was 0.36 +/- 0.10 ml.kg-1 x min-1. After an intramuscular CK administration, the CK systemic availability was 74.1 +/- 21.2% and the half time of absorption was 9.4 +/- 5.7 h, indicating a slow process for CK transit through the lymphatic system. The CK release rate was only significantly increased during the 60-km exercise test. The increase of CK plasma activity was observed after a delay of approximately 5 h and peaked after the end of the race; the estimated CK release rate was 9.92 +/- 2.62 U.kg-1 x h-1 over a mean duration period of 65.8 +/- 15.8 h. With the CK activity of horse striated muscle taken into account, a 60-km race released a quantity of CK corresponding to an equivalent of 18.8 +/- 4.3 g striated muscle. It is concluded that the equivalent amount of damaged muscle may be considered as negligible for a 60-km test and that only very high plasma CK activity levels (at least higher than 10,000 U/l) may provide some evidence of a myolysis.