Recently, the number of women who participate in strenuous exercise has increased significantly. The relationship between delayed menarche and early onset of sports training, and increased incidence of menstrual dysfunction related to athletic activity had led to increased interest. Five basketball players who are among the best players in Japan were subjected to an investigation of endocrinological responses effected by 60 minutes continuous exercise cycle ergometer. The VO2max value was previously examined. Then, the level of load during exercise was established at 60% VO2max in this study. The serum level of estradiol increased significantly in the luteal phase but not so significantly in the follicular phase. Progesterone did not show a significant change during exercise. FSH increased slightly in both phases. However, LH showed a slight increase in the follicular phase and a slight decrease in the luteal phase. On the other hand, prolactin showed a continuous significant increase during exercise in both phases. These data suggested that strenuous daily training leads to a frequent significant increase in prolactin in athletic women and this frequent increase in prolactin may be one of the major factors causing menstrual dysfunction in athletic women. The specialized physician should be more concerned about the managing of athletes who are suffering from menstrual dysfunction.