OBJECTIVE To estimate the adverse effects of glyceryl trinitrate compared with prostaglandin (PG) E2 vaginal tablet for cervical ripening in term pregnancy. METHODS One hundred ten women with term pregnancies referred for induction of labor with Bishop scores of 6 or less were randomly assigned to receive a 500-microg glyceryl trinitrate tablet vaginally (n = 54) or a 3-mg PGE2 tablet vaginally (n = 56), every 6 hours for maximum of two doses. Subjects were sent to the labor ward for amniotomy or oxytocin if their Bishop scores were more than 6 or their cervices were not ripe 24 hours after treatment. Adverse effects, changes in the Bishop scores, progress, and outcomes of labor were assessed. RESULTS Glyceryl trinitrate was associated with fewer episodes of uterine tachysystole (0% versus 9%; P =.02). The median Bishop score after 12 hours was lower in women given glyceryl trinitrate compared with those given PGE2. Adverse effects, including headache and palpitations, were more frequent with glyceryl trinitrate than with PGE2. The cesarean rate was not significantly different between groups. CONCLUSIONS Cervical ripening with glyceryl trinitrate resulted in fewer episodes of tachysystole, but there were significantly more minor side effects. It can be used for cervical ripening at term, but it was not as effective as PGE2.