BACKGROUND: With increasing number of female pilots who fly high-performance aircraft, the gender specific factors have become one of the most important aeromedical considerations. It has been raised that the changes of serum female hormone levels by menstrual cycle may affect vasoregulation and female G-tolerance ultimately. However, the exact relationship between serum female hormone levels and G-tolerance is still unknown. Moreover, well-controlled subjects without taking oral contraceptives are needed to examine the possible effect of menstrual cycle on female G-tolerance. The purpose of this study was to determine how female hormones and menstrual cycle affect female relaxed G-tolerance. METHODS: Eight female subjects were studied to test their relaxed G-tolerance. Seven out of 8 subjects had regular menstrual cycle during the study period. Each subject were exposed to human centrifuge once a week for 4 weeks (one complete menstrual cycle) and blood samplings were performed twice at 2nd and 4th week just before exposure to human centrifuge. The profile of centrifuge training consisted of gradual-onset run (0.1 G/sec) acceleration to the visual endpoint. RESULTS: The changes of 4 different serum female hormone levels failed to show any trend related to relaxed G-tolerance including estrogen. There was no significant difference in relaxed G-tolerance in any menstrual cycle time points. CONCLUSION: We conclude that the changes of female hormone levels and the menstrual cycle have no effect on female relaxed G-tolerance.