The occurrence of seizures in preeclamptic and eclamptic women is considered very unlikely during treatment with the standard dose of continuous i.v. MgSO4. Over 21 mo. 13 patients were encountered who developed seizure activity while receiving i.v. MgSO4. Serum Mg levels at the time of seizure were below the therapeutic range in 11 of the 13 patients, prompting a study of serum Mg levels in preeclamptic patients treated with MgSO4. Random serum Mg samples were obtained from 120 patients treated with i.v. MgSO4 for preeclampsia-eclampsia. The samples were collected 2-48 h after the loading dose and while the patients were taking a maintenance dose of either 1, 2 or 3 g/h. When a maintenance dose of 1 or 2 g/h was used, 98 and 50% of the respective serum Mg values were below levels considered therapeutic by several authors. Therapeutic levels were achieved in all patients receiving a maintenance dose of 3 g/h. The recommended maintenance dose of MgSO4 of 1 g/h was insufficient to prevent the occurrence of eclamptic seizures in some preeclamptic patients.