Diurnal patterns of blood pressure and pressor hormones after 26 weeks gestation were compared in 10 normotensive women, 13 subjects with uncomplicated hypertension, and 8 with biochemical evidence of pre-eclampsia. 4 of the pre-eclamptics showed nocturnal hypertension. Levels of plasma renin activity fell progressively from 9 a.m. to midnight in all three groups, and were significantly lower in pre-eclampsia. Plasma angiotensin II levels fell during the day in normotensives and uncomplicated hypertensives, whereas pre-eclamptics showed loss of this pattern and significantly lower levels than the other groups. Free plasma norepinephrine levels fell during the day and to a similar extent in all three groups. A diurnal pattern for free plasma norepinephrine levels, with lower levels at midnight than during the day, was seen in normotensives and uncomplicated hypertensives but not in pre-eclamptics. Plasma norepinephrine sulphate levels rose from 9.00 a.m. to midnight in normotensives and uncomplicated hypertensives. This pattern was reversed in pre-eclamptics, including 3 of the 4 subjects with nocturnal hypertension. Plasma epinephrine sulphate levels rose progressively through the day, with no significant differences between groups. Failure of plasma angiotensin II or epinephrine levels to fall at night in pre-eclampsia may contribute to nocturnal hypertension in subjects with increased vascular reactivity. Suppression of plasma renin activity and angiotensin II levels suggests that perhaps as yet unidentified pressor mechanisms are involved in pre-eclampsia.