The menstrual cycle is an integral part of life in woman and associated with pronounced physiological changes. There is widespread agreement about the hormonal changes that occur during the menstrual cycle, but studies on cardiovascular functioning during different phases of menstrual cycle are more controversial. The purpose of the present study was to evaluate the influence of the normal menstrual cycle on heart rate (HR) and heart rate variability (HRV). Fourteen healthy regularly cycling woman (age 19-24 years) were studied during the follicular phase and luteal phase over three month. The time schedule for the luteal phase was from days 3 to 7 before the first day of bleeding, and for the follicular phase was from days 7 to 11 after the first day of bleeding [1]. HRV was recorded after 10-minute supine rest for 5 minutes while subjects were resting in a supine position [2]. The values were compared by Mann-Whitney test (U). Mean values of HR, VLF, LF, HF and LF/HF for the different phases are presented in Table 1 We found pronounced changes in HR during the menstrual cycle with a minimum in the follicular phase and maximum in the luteal phase (U=1008.00; p<0.0001). Power spectral analysis of HRV showed that the HF component of HRV, which reflects only parasympathetic activity, decreased more during the luteal phase than follicular phase (U=1557.00; p=0.0016), suggesting that parasympathetic nerve activity increases in this phase. The LF/HF ratio, which reflects the balance of autonomic nerve activities, in the luteal phase was significantly higher than that in the follicular phase (U=1818.50; p=0.04), indicating that sympathetic nerve activities in the luteal phase became predominant. VLF and LF did not show any significant change during the different phases of menstrual cycle. This findings indicate that sympathetic nervous activity in the luteal phase is significantly greater than in the follicular phase whereas parasympathetic nervous activity is predominant in the follicular phase. Although the mechanism by which the menstrual cycle affects the autonomic nervous system is unclear, some endogenous female sex hormones may be responsible for these changes in the cardiac autonomic innervation.