OBJECTIVE: Our purpose was to determine whether seizure prophylaxis with magnesium sulfate prolongs induction of labor in women with pregnancy-associated hypertension. STUDY DESIGN: Women with a singleton pregnancy in vertex presentation between 32 and 42 weeks' gestation who required induction of labor for either preeclampsia, preeclampsia superimposed on chronic hypertension, or transient hypertension were randomized to receive either magnesium sulfate or phenytoin (Dilantin) for seizure prophylaxis. Women with rupture of the membranes, spontaneous contractions resulting in cervical change, or an initial cervical examination showing >2 cm dilatation and 50% effacement were excluded. RESULTS: Fifty-four women were randomized to receive either magnesium sulfate (n = 28) or phenytoin (n = 26). The two groups were similar for selected maternal antepartum, intrapartum, and postpartum characteristics that have been associated with a prolongation or failure of labor induction. The two groups were also similar for the interval from the start of induction to (1) the first examination ≥5 cm cervical dilatation and (2) delivery and in the frequency of women requiring cesarean delivery. CONCLUSION: Compared with phenytoin, magnesium sulfate seizure prophylaxis in women with pregnancy-associated hypertension does not prolong the induction of labor nor does it result in an increase in cesarean deliveries. © 1995.