The purpose of this study was to investigate moderate endurance exercise-induced changes in oxidative stress in different phases of the menstrual cycle. We used saliva samples which can be collected non-invasively. The resting oxidative stress levels determined by oxidation-reduction potential (ORP) were significantly higher in the menstrual phase than the follicular and luteal phases. The ORP significantly increased 30 minutes after exercise started and at the end of the 60 minute exercise session in the follicular phase. The menstrual cycle may affect oxidative stress levels at rest among young women, whereas moderate endurance exercise is likely to increase oxidative stress in the follicular phase.