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± 0.16mmol/l (means± s.d.);P< 0.05] and increased after treatment with Mg2+to 1.19± 0.24mmol/l. Lowered cellular Mg2+concentrations in pre-eclampsia may contribute to the development of hypertension in this disorder.