1. One hundred and five primigravidae were followed sequentially at 4-weekly intervals starting at gestational week 31. They were seen again at 6 weeks post partum.

2. At each visit measurements were made of blood pressure as well as of leucocyte and erythrocyte sodium and potassium content.

3. Eighty-five subjects completed the study. Seven developed pre-eclampsia.

4. In both controls and patients who developed preeclampsia, leucocyte and erythrocyte sodium content increased with gestational age and fell post partum. These changes were of greater magnitude in the patients with pre-eclampsia. Cell potassium fell in both groups, but to a greater extent in patients with pre-eclampsia.

5. These changes in cell sodium paralleled those in blood pressure in both groups.

6. These data suggest that the excessive blood pressure changes in pre-eclampsia might be related to similar changes in cell sodium content.

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