Pulmonary function was studied in ten preeclamptic women in labor (mean gestational age 38.1 +/- 0.9 weeks measured from the last menstrual period) receiving continuous intravenous (IV) infusions of magnesium sulfate. Baseline maximal inspiratory pressure, maximal expiratory pressure, functional vital capacity, and forced expiratory volume at 1 second were measured immediately before a 6-g IV loading dose of magnesium sulfate and 2 hours after the initiation of a continuous 2-g/hour infusion of magnesium sulfate. Serum magnesium levels were measured at the same time pulmonary function tests were performed. All values are reported as the mean +/- standard deviation. The maximal inspiratory pressure, an indicator of generalized respiratory muscle weakness, decreased from a baseline value of 26.2 +/- 7.7 to 19.4 +/- 6.3 cm H2O (P less than .05). The maximal expiratory pressure, an indicator of expiratory muscle strength, decreased from a baseline value of 30.6 +/- 9.2 to 25.2 +/- 7.1 cm H2O (P less than .005). The functional vital capacity decreased from a baseline value of 3.37 +/- 0.49 to 3.19 +/- 0.73 L, and the forced expiratory volume at 1 second decreased from a baseline value of 2.61 +/- 0.58 to 2.36 +/- 0.68 L at 2 hours (P less than .05). The mean serum magnesium level was 1.7 +/- 0.2 mg/dL before the administration of the IV loading dose and 4.51 +/- 0.67 mg/dL 2 hours after initiation of the continuous infusion. Our results demonstrate a significant decrease in pulmonary function tests in term preeclamptic patients receiving magnesium sulfate for seizure prophylaxis.(ABSTRACT TRUNCATED AT 250 WORDS)