Serum magnesium levels, as well as magnesium content of red blood cells and peripheral mononuclear cells, were examined in 31 pregnant women in their third trimester. Ten were preeclamptic; chronic hypertension was found in 10, and 11 were normotensive. Magnesium serum levels were 1.2 ± 0.1,1.2 ± 0.1 and 1.3 ± 0.1 mEq/1 in the normotensives, chronic hypertensives and pre-eclamptics, respectively. Red blood cell magnesium concentration was 3.4 ± 0.4, 3.7 ± 0.7 and 3.5 ± 0.5 mEq/1, and mononuclear magnesium content was 37.9 ± 30.6, 27.6 ± 15.9 and 30.2 ± 25.7 fg/cell in the same groups, respectively. These changes were not statistically significant. The results do not support the hypothesis that magnesium deficiency is involved in the pathophysiology of preeclampsia.