It is well established that prolactin release during exercise is one of the important factors in exercise-induced menstrual dysfunction. The purpose of this study is to clarify the mechanisms of prolactin release during exercise. Ten female athletes measured their BBT every morning. They performed incremental exercise on a cycle ergometer, with or without naloxone, on the 5th to 8th days of the follicular phase. Three minutes before the exercise, 0.4mg of naloxone was injected intravenously and a further 1.6mg/hr of naloxone was continuously infused during exercise. Blood samples were collected after 60 minutes bed rest (Rest), at the time when the heart rates reached 150 bpm (Submax), the point of exhaustion (Max) during exercise and after 60 minutes bed rest following exercise (After 1hr). The levels of prolactin in serum, dopamine, beta-endorphin. VIP and ACTH in the plasma were measured. Whereas prolactin increased significantly at Submax (p less than 0.05) and Max (p less than 0.001), the increase in prolactin was suppressed by the administration of naloxone (p less than 0.05). Dopamine showed no remarkable change during exercise, with or without naloxone. There were significant increases in beta-endorphin at Max (p less than 0.001), VIP at Submax and Max (p less than 0.001), but these increases were suppressed by the administration of naloxone (p less than 0.001). ACTH which had markedly increased at Submax (p less than 0.025) and Max (p less than 0.001) showed a slight tendency to decrease following the administration of naloxone, but there were no significant differences in both groups.(ABSTRACT TRUNCATED AT 250 WORDS)