Blood pressure was monitored continuously and intraarteriously for 24 hours in healthy and pre-eclampsia pregnant women. The monitoring system almost excluded sources of error. The 24-hours-rhythm revealed a blood pressure decrease of about 20 Torr systolically and diastolically between 2.00 a.m. and 5.00 a.m. in normal pregnancy. This decrease after midnight was diminished in mild cases of pre-eclampsia. Hypertensive peaks were present after midnight in severe pre-eclamptic toxemia even under medication. Measuring of the blood pressure in pre-eclampsia patients during nighttime from 0.00 a.m. until 5.00 a.m. seems to be advisable in order to identify unstabilized hypertensive peaks.