We hypothesized that treatment of pregnant mice with the progesterone receptor antagonist RU486 might cause preterm labor and result in the delivery of live pups. We also hypothesized that RU486 administration would alter prostaglandin production by decidual explants taken from these pregnancies. C3H/HeN females mated with C57BI/6 males were injected with a single s.c. dose of RU486 on Days 12-14 of gestation. Three doses were tested (50 μg, 150 μg, and 250 μg), and the mice were observed for evidence of delivery. The time course of delivery was determined in a second experiment using 150 μg of RU486, and care was taken to observe the condition of the delivered pups. In a third experiment, mice were killed when delivery commenced after injection with 150 μg of RU486, and decidual explants were prepared. Controls that had received injections of vehicle were killed at the same time, and decidual explants were established. Media were removed after 24 h and analyzed for prostaglandin E2 (PGE2) and prostaglandin F(2α) (PGF(2α)) by RIA and for interleukin 6 (IL-6) by ELISA. Two of 3 mice given 50 μg of RU486 delivered 16 pups prematurely. All 3 mice given 150 μg of RU486 delivered 22 pups prematurely, and 2 of 3 given 250 μg of RU486 delivered 12 pups prematurely. Of mice treated with 150 μg of RU486, none delivered within 12 h; 2 of 7 delivered within 15 h; and 6 of 7 delivered within 22 h. All pups appeared to be healthy, with no evidence of placental infarction or death. PGE2, PGF(2α), and IL-6 production by decidual explants was significantly greater in tissues taken from RU486-treated mice (n = 6) than in controls (n = 3). In summary, RU486 reliably induced preterm birth of the mice within 24 h after s.c. injection. This was associated with increased decidual prostaglandin and cytokine production and thus may mediate preterm labor. Inducing preterm birth with RU486 in a mouse model may be useful in investigations of the mechanism(s) of preterm labor.