Measurement of 6 ketoprostaglandin F1 alpha was made by radioimmunoassay during the first 3 days of life in 33 infants with respiratory distress syndrome who were subjects in a double blind controlled trial of ethamsylate for the prevention of intraventricular haemorrhage. Levels of 6-ketoprostaglandin F1 alpha were significantly lower on the first and second days of life in babies receiving ethamsylate. There was a reduction in the incidence of intraventricular haemorrhage in the treated group. High levels of prostacyclin metabolite are found in babies who develop haemorrhage, and reduction of prostacyclin synthetase activity may be the mode of action of this drug in vivo.