Thirty-four sick neonates with major duct dependent cardiac defects were given short term (1 h-408 h) intravenous infusions of prostaglandin E1 (alprostadil) in doses varying between 0.1 micrograms/kg/min (starting dose) and 0.01 micrograms/kg/min. The aim of the study was to establish an effective and safe regiment that could be initiated after clinical diagnosis of a severe duct dependent cardiac defect, whose clinical course would be adversely affected by ductus closure. After an initial dosage of 0.1 micrograms/kg/min, effective clinical improvement was achieved in 28 infants (82%). In all 28 responders, the effect was maintained at a reduced dosage of 0.05 micrograms/kg/min, and in 21 a low dosage of 0.01 micrograms/kg/min was effective. Side effects, which occurred in 21 infants (62%), were reversible and dose related, and no serious side effects were noted at a dosage of less than 0.05 micrograms/kg/min. A starting dose of 0.05 micrograms/kg/min with subsequent reduction is recommended, but in case of institution of treatment before transfer to a pediatric cardiac centre a lower starting dose of 0.01 micrograms/kg/min may be preferred.