Five women and three men (aged 24-43 yr) performed maximal eccentric contractions of the elbow flexors (for 20 min) on three occasions, spaced 2 wk apart. Muscle pain, strength and contractile properties, and plasma creatine kinase (CK) were studied before and after each exercise bout. Muscle tenderness was greatest after the first bout and thereafter progressively decreased. Very high plasma CK levels (1,500-11,000 IU/l) occurred after the first bout, but the second and third bouts did not significantly affect the plasma CK. After each bout the strength was reduced by approximately 50% and after 2 wk had only recovered to 80% of preexercise values. Each exercise bout produced a marked shift of the force-frequency curve to the right which took approximately 2 wk to recover. The recovery rate of both strength and force-frequency characteristics was faster after the second and third bouts. Since the adaptation occurred after the performance of maximal contractions it cannot have been a result of changes in motor unit recruitment. The observed training effect of repeated exercise was not a consequence of the muscle becoming either stronger or more resistant to fatigue.