Newborn rats delivered by caesarean section were given subcutaneously verapamil hydrochloride (VER), a Ca-channel blocker, (1) immediately or (2) 180 min after delivery. The diameter of the ductus arteriosus (DA) of the newborn pups was calibrated at 30, 60 and 90 min after the VER-administration. (1) The DA calibers of the pups given 0.1, 1.0 mg/kg VER immediately after delivery remained significantly larger than those of control pups in a dose-dependent manner for 30 min after treatment. In the 1 mg/kg group, the enlargement of the DA was prolonged until 90 min after treatment. (2) In untreated pups, the DA completely closed by 180 min after caesarean delivery. The closed DA was not affected after 1 mg/kg VER was given at 3 hours after delivery. It was concluded that, VER inhibits the spontaneous constriction of the DA, suggesting that the increase of intracellular Ca2+ concentration may play an important role on the spontaneous closure of the DA in newborn rats.