Two to four stems ofLaminariawere inserted into the cervixes of seven healthy women at second trimester. Fortyeight hours later, theLaminariain each patient were replaced with a rubber balloon (100 mL capacity) that was inflated with 150 to 180 mL of physiologic saline solution, and traction of 800 to 1000 g was applied for an additional 24 hours. During Laminaria application, marked cervical softening was noted in all cases, and weak but definite uterine contractions were initiated in three women. During the balloon application regular uterine contractions were initiated or increased in all patients, with three late second trimester patients aborting within 24 hours. Light and electron microscopic studies of the decidua parietalis obtained during apparent uterine activity and after abortion revealed normal cell morphology. A significant increase in amniotic fluid F prostaglandin was noted after the balloon treatment (P< .05). The authors conclude that uterine stretching induces uterine activity and a rise of prostaglandin in amniotic fluid without disfunction of decidual cells.