Maturation of fetal ductus arteriosus is associated with increased constriction in response to maternally administered indomethacin. Recently retinoic acid has been shown to be important in development of the fetal ductus arteriosus. To determine whether retinoid might be of value in the treatment of patent ductus arteriosus in premature infants, we studied the response of fetal ductus arteriosus to indomethacin with and without pretreatment with vitamin A (1 mg (3000 IU)/kg, intramuscular injection) in near-term and preterm rats. Maturation of the ductus arteriosus was studied by measuring the inner diameters of the ductus arteriosus (D) and main pulmonary artery (P) to get D/P ratio 4 h after orogastric administration of 1 mg/kg indomethacin. D/P was 1.0 in the fetus before administration of indomethacin. In near-term fetuses on the 21st d without vitamin A, D/P decreased to 0.54 with indomethacin, whereas it decreased to 0.27 (p< 0.05) in those with pretreatment with vitamin A on the 19th and 20th d. In preterm fetuses on the 20th d without pretreatment with vitamin A, D/P decreased to 0.82 with indomethacin, whereas it decreased to 0.66 (p< 0.05) in those with pretreatment with vitamin A on the 19th d. It is concluded that maternally administered vitamin A accelerates maturation of the ductus arteriosus in fetal rats.Abbreviations: DA,ductus arteriosus;D/P,ductus arteriosus/main pulmonary inner diameter ratio;PDA,patent ductus arteriosus