Flufenamic acid (FA), an inhibitor of the synthesis and action of prostaglandins, was administered to 18 women with preterm labor during the 28th to 36th week of gestation. In 15 patients delivery was postponed, the mean admission/delivery interval being 21.5 days. 2 patients with cervical dilatation of 4 cm delivered within 48 h despite medication. The peripheral plasma levels of 15-keto-13,14-dihydroprostaglandin F2α (KH2 PG2α) was high on admission (216 ± 24 pg/ml, mean ± SEM), declined by 50% within 2 h of instituting treatment and remained near the normal level seen in 13 controlled women after the 24th hour.