The pattern of changes in plasma concentrations of melatonin was followed in 7 initially untrained young women subjected to periodic acute exercise testing during the course of an 8 weeks' progressive aerobic exercise training program. Training comprised cycle ergometry 2 days/week and running 4 days/week for increasingly prolonged periods of intense exercise eliciting 85% of maximum heart rate. Acute exercise tests consisted of one-hour graded submaximal endurance rides on a bicycle ergometer. After a mock ride for familiarization purposes, three test rides were conducted during the early follicular phase of three consecutive menstrual cycles at the beginning, middle and end of the training program. Blood was sampled atraumatically before the rides, at their conclusion and 30 minutes into recovery. During the rides, the plasma concentrations of melatonin rose significantly above baseline control values. Exercise-enhanced melatonin levels may contribute to impaired reproductive function in women engaging in endurance sports.