Four normal subjects performed a 20 min step test using a step of the same relative height. During the test the quadriceps muscle of one leg contracted concentrically throughout by stepping up, while the contralateral muscle contracted eccentrically by controlling the step down. Thus both muscles performed the same amount of work. Three subjects had bilateral needle biopsies just prior to exercise. All four had bilateral biopsies immediately after exercise, and 24-48 hours later when the muscles which had contracted eccentrically were painful. The samples were examined by light and electron microscopy. No abnormalities were seen in pre-exercise samples nor after exercise in muscles which had contracted concentrically. The muscles which had contracted eccentrically showed some damage immediately after exercise. In the samples taken 24-48 hours after exercise the damage was more marked and involved a greater percentage of fibres. In view of the known differences between these types of contractions it is suggested that the initial damage is mechanically induced. The exacerbation of damage with time could be due to mechanical or chemical factors.