We tested the idea that real-world situations, such as the highly strenuous exercise involved in marathon running, that impose extreme physical demands on an individual may result in neurohormonal changes that alter the functioning of memory. Marathon runners were given implicit and explicit memory tasks before or immediately after they completed a marathon. Runners tested immediately upon completing the marathon showed impairment in the explicit memory task but enhancement in the implicit memory task. This postmarathon impairment in explicit memory is similar to that seen with amnesic patients with organic brain damage. However, no previous studies have shown a simultaneous enhancement in the implicit memory task, as shown by the marathon runners in the present study. This study indicates that human memory functioning can be dynamically altered by such activities as marathon running, in which hundreds of thousands of healthy normal individuals routinely partake.