Oestrogen is considered to be cardioprotective, with the incidence of cardiovascular disease increased in post-menopausal women (Rosano et al., 2006). Oestrogen acts, in part, by modifying the autonomic nervous system (ANS) (Saleh and Connell, 2007). Heart rate variability (HRV) gives a non-invasive indication of ANS control. To date, little attention has been paid to ANS responses to stress, in particular mental stress (MS) in women during different phases of the menstrual cycle and also post-menopause. The aim of this study was to identify if menstrual phase altered vagal activity in three conditions: a) baseline; b) during MS and c) in the recovery from MS. Following ethical approval, thirty three participants, 34.9±11.9 years, were placed into four groups by the following criteria: luteal (from day 20-25) (n=7), follicular (day 12-18) (n=9), menses (day 1-8) (n=8) and postmenopausal (no menstruation for more than 2 years) (n=9). After 15 minute rest in supine position, HR was recorded using a Suunto HR monitor for 10 minutes (baseline); immediately followed by 5.30 min during MS (forward digit span); and 10 minutes of recovery. The HR data were analysed using Kubios HRV analysis software, (University of Kupio, Finland). Two indices of vagal activity were calculated: the root mean square of successive differences (RMSSD); and natural logarithm of high frequency power (frequency domain) (lnHF). One-way ANOVA with post hoc comparisons was used to compare differences in HRV parameters between the four groups Repeated measures ANOVA was used to assess within-group changes from baseline. At baseline, the 3 groups (excluding post-menopause) were not significantly (NS) different in any of the indices, although Follicular group had the lowest HR and highest vagal markers. Post-menopause group had significantly higher HR and lower values for RMSSD, compared to Follicular group and lower lnHF compared to the other 3 groups. MS induced significant increases in HR from baseline (7.8±6, 7.6±5, and 3.4±1 bpm in all groups (p≤0.05) except Menses 5.1±8 bpm, NS). This was accompanied by reduction in vagal activity in all groups (not significant). During MS, the increase in HR for post-menopause group was significantly smaller compared to Follicular group. There were no significant differences between any of the groups in recovery from MS. These results indicate that at baseline, during and in recovery from MS, HRV indices during different phases of menstrual cycle are not significantly different. This suggests that in women overall cardiac autonomic control during MS does not appear to be altered by the different phases of menstrual cycle. The use of post-menopausal women should be considered carefully when conducting research into autonomic control when comparing to non post-menopausal groups.