Tylose gel containing 400 micrograms prostaglandin E2 in 3 ml gel was injected into the cervical canal of 20 patients with high-risk pregnancy and indication for induction of labor, but with unfavorable cervix. Ten were studied after the first gel application, five during repeat injection, and five after application of gel without PGE2. Blood samples were drawn serially during the first 8 hours for determination of oxytocin and 13,14-dihydro,15-ketoprostaglandin-F2 alpha (PGFM). The PGE2 gel increased the Bishop score within 8 hours in all patients; in half of them, artificial rupture of the membranes could be performed and labor induced without further gel application; in the others, it was repeated every 8 hours until a Bishop score of greater than or equal to 8 was achieved. Fourteen of the 15 PGE2-induced patients delivered vaginally. Mean PGFM levels did not increase significantly during the 8 hours of observation, but in patients who responded with rapid progression, an increase was seen after cervical dilation was 6 cm or more. The mean oxytocin levels increased within 60 minutes after PGE2 application and were increased for the remaining observation period. Application of inactive gel had no effect on cervical ripening nor on oxytocin or PGFM levels.