Variations in cardiovascular functioning during the 'normal' menstrual cycle have been little researched. Resting-blood pressures, resting-heart rate, rate-pressure product (RPP) and a derived index of fitness (Schneider Index) were monitored throughout natural, hormonally defined menstrual cycles. Volunteers were 26 women (20-48 years) who had regular (25-35 days) cycles. Their blood pressures and heart rate (at rest and according to Schneider's protocol) were measured at the same time daily (Monday-Friday) for 5 weeks. Daily, early morning-urine samples were assayed for sex hormones enabling accurate definition of cycle phase for each woman. Resting systolic-blood pressure was significantly higher in the ovulatory phase (P < 0.05) than in the follicular or luteal phases, but resting-diastolic pressures did not differ significantly between phases. Resting-heart rate was significantly higher in both ovulatory (P < 0.01) and luteal (P < 0.01) phases than in the menstrual and follicular phases. The Schneider Index was higher during the follicular phase than during the ovulatory (P < 0.005) or luteal (P < 0.01) phases, the RPP was higher during the ovulatory phase than during the bleeding (P < 0.05) and follicular (P < 0.005) phases. These findings provide a pattern of menstrual cycle-related variation in cardiovascular functioning that can be related to established actions of the ovarian steroids.