During one training year studies of five long distance, endurance rides were carried out. Blood samples were obtained at the start and finish, and plasma concentrations of urea, bilirubin and enzyme activities of tau-GT, LDH, CK, and AST were determined. As a result of the physical effort there were significant changes in these parameters, but they were not similar at each ride. Except for bilirubin, all decreased in concentration, and most marked were tau-GT and LDH.