This study aimed to identify the links between the use of prohibited performance-enhancing substances and the changing characteristics of athletes' activity within the framework of situated action and dynamical investigations. The changes in diverse components of activity (e.g. perform- ance, forms of training, modes of involvement) of 10 elite track and field athletes (5 with 2-year sus- pensions) were compared. Data were collected by enquiries and structured and self-confrontation interviews. Results showed that during the use of prohibited substances, doping athletes appeared (1) to be closed to all external environmental offers except training and performance, (2) to experience changes in their sporting results and (3) to be experiencing disturbances in their lives. Doping appeared after (4) a specific number of years of sporting activity (i.e. 17 years), (5) a specific path had been followed (i.e. a shorter time spent in "open focus" during the development of activity), (6) 2 years of regular legal substance use, (7) a change in training and (8) a period of personal distress. A signature of doping activity was identified in relation to suffering athletes. Several initial conditions that lead to prohibited substance use were extracted and are utilized for educational programmes.