Plasma and intra-erythrocytic magnesium concentrations were determined in 27 patients with pre-eclampsia and in 22 healthy pregnant women. In the pre-eclamptic women, the Mg2+ concentrations were measured before and after treatment with Mg2+ salts and after delivery. The plasma Mg2+ concentration was not significantly different in the pre-eclamptic and the healthy pregnant women. The intra-erythrocytic Mg2+ concentration before treatment with Mg2+ was significantly lower in the pre-eclamptic patients than in the healthy pregnant women [1.33 +/- 0.29 versus 1.01 +/- 0.16 mmol/l (means +/- s.d.); P less than 0.05] and increased after treatment with Mg2+ to 1.19 +/- 0.24 mmol/l. Lowered cellular Mg2+ concentrations in pre-eclampsia may contribute to the development of hypertension in this disorder.