The effect of prostaglandins F2alpha, E1 and of 7-oxa-13-prostynoic acid on the newborn rabbit ductus can be studied using the whole-body freezing technique. PGF2alpha and PGE1 were able to re-open the closing ductus arteriosus in adequately oxygenated animals. PGF2alpha administration was accompanied by a strong physical reaction in the rat but less in the rabbit. PGE1 had sedative effects in both animals. A prostaglandin antagonist, 7-oxa-13-prostynoic acid had no effect on normal ductal closure nor did it counteract the effects of PGF2alpha and PGE1. The role of prostaglandins in homeostasis during the fetal and newborn period may be to modify ductal tone.