This study was undertaken to determine if blood pressures (BP) assessed by routine sphygmomanometry and 24 h ambulatory monitoring (ABPM) alter throughout the normal menstrual cycle or in the cycle of women using oral contraceptive pills (OCP), and the interrelationships between urinary sodium (Na) and potassium (K) excretion and ABPM throughout the menstrual cycle. Eleven women with a normal ovulatory cycle (ovulatory) and ten age-matched women taking an oral contraceptive pill (OCP) were studied three times in random order during their menstrual cycle, within days 1 to 5, 13 to 16, and 25 to 28. Twenty-four hour urine Na, K, and creatinine (Cr) excretion and serum Na, K, Cr, cortisol, estradiol, progesterone and plasma renin, angiotensinogen, and aldosterone concentrations were measured. BP was measured by a mercury sphygmomanometer and by 24 h BP (Accutracker II). On days 1 to 5, daytime systolic BP was higher in OCP [mean: 123 mm Hg, 95% confidence interval: 117, 128] than ovulatory women [114 mm Hg (109, 118); P = .011] though daytime diastolic BPs were similar [OCP: 71 (68, 75), ovulatory: 69 (66, 72)]. This difference in daytime systolic BP between groups was also apparent at both of the other stages of the menstrual cycle. Nighttime systolic BPs were significantly higher in OCP users on days 13 to 16 (P < .05) and days 25 to 28 (P < .01). In women taking OCPs, daytime ABPM for days 1 to 5 were higher than their office readings by 15 (7,23)/11 (7,15) mm Hg (P = .001), whereas office and ABPM readings were similar in ovulatory women. This pattern was evident at all three stages. There was no significant change in BP throughout the menstrual cycle within either group, and no correlation between urine Na or K and BP. Systolic BPs are higher throughout the menstrual cycle in women who take OCPs than in ovulatory women but this difference is only detected when ambulatory blood pressure is assessed. Blood pressure does not change subsequently in either ovulatory or OCP-taking women throughout the menstrual cycle.