Although it is generally accepted in the sporting world that adrenocorticotropic hormones (ACTH) and corticosteroids enhance maximal performance, this claim has never been scientifically corroborated. In a counterbalanced, double-blind design, 1 mg ACTH or placebo was injected into 16 professional cyclists. They cycled for 1 h on a bicycle ergometer at a submaximal level, defined as 60% of maximal performance on a pretest with a load increase of 50 W per minute. After 1 h, load was increased by 10 W per minute until exhaustion. No increase of maximal performance was observed with ACTH, either on the day of drug intake, or on the following day, although substantial increases were measured in physiological variables such as cortisol, glucose, and white corpuscle concentrations. Feelings of fatigue, which were continuously self-rated, were diminished only during submaximal performance. The present technique of systematically distinguishing between physiological, psychological, and performance measurements could help in explaining the persistent belief in the performance enhancing properties of ACTH and other doping substances.