Furosemide increases prostaglandin production and may be associated with patent ductus arteriosus (PDA). We aimed to clarify the in vivo ductus-dilating effects of furosemide in neonatal rats. Near-term rat pups delivered by a cesarean section were housed at 33 degrees C. After a rapid whole-body freezing, the DA diameter was measured using a microscope and a micrometer. Pregnant rats (gestational day 21) were s.c. injected with furosemide 4 h before delivery, and the neonatal DA was examined 0, 15, 30, 60, and 120 min after birth. Furosemide was also s.c. injected into 60-min-old rats and the DA diameter was examined 30, 60, and 120 min later. The control rats showed a rapid postnatal DA constriction (diameter: 0.80 and 0.08 mm at 0 and 60 min after birth, respectively). Prenatally administered furosemide delayed postnatal DA closure (0.36 mm at 60 min after birth). Furosemide injection in 60-min-old rats dilated the constricted DA at 60 min (0.25 versus 0.02 mm in the controls). Indomethacin inhibited furosemide-induced DA dilatation. Furosemide delays DA closure and dilates the constricted DA in neonatal rats. If furosemide has similar effects in human preterm neonates, caution may be warranted in its use in the treatment of infants with PDA.