Serum creatine phosphokinase (CPK), aspartate aminotransferase (AST), and alanine aminotransferase (ALT) activities were assayed in 15 healthy but untrained subjects (US) and seven Olympic rowers (OR). Blood samples were obtained at rest, and five and 60 minutes after an ergometer test, during which cardiorespiratory data were obtained. OR demonstrated higher resting CPK (p < 0.001) and AST (p < 0.05) levels than the US. CPK and AST reached their highest levels five minutes after the end of the ergometer test in both groups, whilst peak ALT was reached five and 60 minutes post-test in the US and OR respectively. Fitness parameters and serum enzyme levels were better correlated in the OR group. It is concluded that: (1) exercise temporarily increases the activities of serum AST, ALT, and CPK, (2) increased physical fitness has no bearing on resting serum ALT enzyme levels, although this was not the case with AST and CPK, and (3) exercise duration rather than fitness levels seems to be related to serum CPK, AST, and ALT activities.