The menstrual cycle provokes several physiological changes that could influence autonomic regulatory mechanisms. We studied the carotid-cardiac baroreflex in ten healthy young women on four occasions over the course of their menstrual cycles (days 0-8, 9-14, 15-20 and 21-25). We drew blood during each session for analysis of oestrogen, progesterone and noradrenaline (norepinephrine) levels, and assessed carotid-cardiac baroreflex function by analysing R-R interval responses to graded neck pressure sequences. Oestrogen levels followed a classical two-peak (cubic) response, with elevated levels on days 9-14 and 21-25 compared with days 0-8 and 15-20 (P=0.0032), while progesterone levels increased exponentially from days 9-14 to days 21-25 (P=0.0063). Noradrenaline levels increased from an average of 137 pg/ml during the first three measurement periods to 199 pg/ml during days 21-25 (P=0.0456). Carotid-cardiac baroreflex gain and operational point were not statistically different at any of the time points during the menstrual cycle (P> or =0.18). These findings are consistent with the notion that beat-to-beat vagal-cardiac regulation does not change over the course of the normal menstrual cycle.