OBJECTIVE To measure the apparent volume of distribution (AVOD) for magnesium (Mg) in preeclampsia and preterm labor and determine if a standard 4 gm loading dose of magnesium sulfate (MgSO4) is sufficient to attain therapeutic levels. METHODS Twenty-five patients with preeclampsia and 25 with preterm labor received 4 g of MgSO4 intravenously over 15 minutes. Serum Mg levels were determined before and one minute after loading and the AVOD for Mg was calculated. Stepwise linear regression with AVOD as the dependent variable was performed and comparisons between the groups were made. RESULTS Preeclamptics were heavier, had greater surface areas, and presented at a later stage of pregnancy than did patients with preterm labor. Despite these differences AVOD did not differ between the groups. Predose magnesium levels were slightly higher in the preeclamptic group (p = .04). Post-loading levels were nearly identical due to similar AVOD's and, because of the lower levels required for seizure prevention as opposed to tocolysis, were therapeutic 88% of the time in preeclampsia but only 12% of the time in preterm labor (p < .001). Multivariate analysis revealed that only ideal body weight, degree of underweight, and current therapy with betamimetics were significantly related to AVOD. CONCLUSIONS AVOD was found to be similar in preeclamptic and preterm labor patients. A 4 g loading dose of MgSO4 is usually adequate to achieve therapeutic levels in preeclampsia but not in preterm labor.