Muscle changes have been examined in 16 normal subjects (eight female) after both a 20-minute and a prolonged step test. Stepping differs from most exercise tests in that it involves eccentric contractions (negative work) in which the active muscle is lengthened. Plasma creatine kinase (CK), muscle force, contractile properties, and tenderness in the quadriceps were measured for up to 9 days after the exercise. Muscle tenderness was experienced only in the muscles that had performed eccentric contractions (i.e., stepped down). All subjects showed some early rise in CK (less than 400 IU/liter) but eight (both male and female) showed a much greater response (up to 34,500 IU/liter) which took a long time to reach peak levels (4-5 days after stepping). It is suggested that eccentric contractions involved in this form of exercise result in some particular form of muscle damage which, in susceptible subjects, may initiate changes giving rise to a large delayed release of muscle enzymes.