Six highly-trained male swimmers completed a maximum work capacity tethered swim and a 1-h continuous tethered swim at ~ 70% V O 2max in order to evaluate total serum creatine kinase and CK-MB isoenzyme changes. Venous blood obtained before, 5 min post-, 6 h post-, and 24 h post-exercise was analyzed for total serum CK (kinetic UV method, normal = < 100 U/l) and CK-MB isoenzyme (quantitative electrophoretic technique, normal = < 5 U/l). V O 2max averaged 4.59 ± 0.28 l/min, with a mean total work time of 24.5 min to achieve maximum capacity. Mean resting total CK was 100.5 ± 15.8 U/l. Compared to rest, neither swim bout produced a significant ( p > 0.05) elevation in mean total creatine kinase. No CK-MB isoenzyme was observed in any post-exercise blood sample. Swimming, performed by highly-trained swimmers at high levels of intensity or for prolonged durations, may not impose sufficient degrees of trauma producing muscular stress. Therefore, the structural integrity of the cell membrane is maintained and the loss of intracellular creatine kinase to the bloodstream prevented.