Tolazoline hydrochloride was given as a pulmonary vasodilator to 20 preterm infants with severe hyaline membrane disease requiring mechanical ventilation who developed persistent severe hypoxaemia. The drug was only given when deterioration persisted despite changes in ventilation techniques and thus there were no control infants. A significant improvement in arterial oxygenation was observed, particularly in those infants without intracranial haemorrhage or pneumothroax. Nine of 10 infants who had an obvious clinical response to tolazoline survived, whereas only 2 survived out of 10 who failed to respond, both being subsequently shown to have pneumothoraces. There appears to be a place for the use of tolazoline in a severely hypoxaemic infant with hyaline membrane disease who is being ventilated and in whom arterial oxygenation cannot be improved by further increase in the inspired oxygen concentration or by alteration of ventilator settings, provided pneumothorax has been excluded.