Circulating prostaglandin metabolites 13,14-dihydro-15-keto-prostaglandin F2 alpha and the bicyclo derivative of prostaglandin E2 were measured in maternal plasma by radioimmunoassay after transabdominal cervicoisthmic cerclage and after transvaginal cerclage (Shirodkar and McDonald procedures) performed in the first and second trimesters. Statistically significant elevations in prostaglandin E2 metabolite or 13,14-dihydro-15-keto-prostaglandin F2 alpha occurred after transabdominal cervicoisthmic and transvaginal cerclage; they returned to control levels within 6 to 24 hours after surgery and were associated with good fetal outcome. Increases in 13,14-dihydro-15-keto-prostaglandin F2 alpha were proportionately greater than in prostaglandin E2 metabolite. Mean basal levels and the rise in prostaglandin metabolites were not related to cerclage type, trimester of pregnancy, or cervical status (dilatation less than or equal to 3 cm; effacement less than or equal to 60%). Highest basal and postcerclage 13,14-dihydro-15-keto-prostaglandin F2 alpha and prostaglandin E2 metabolite levels were associated with advanced cervical changes, uterine irritability, membrane prolapse or rupture, and premature delivery. Routine administration of prostaglandin synthetase inhibitors is not indicated for transvaginal cerclage or transabdominal cervicoisthmic cerclage; plasma prostaglandin metabolite levels may identify patients not suitable for cerclage.