Intravenous indomethacin was given to 36 neonates with a patent ductus arteriosus who where receiving ventilatory support for respiratory distress syndrome. Permanent closure of the ductus arteriosus occurred in 21 (58%) infants and in this group the mean 24 hour plasma indomethacin concentration was 0.37 micrograms/ml. Partial success was achieved in 6 (17%) infants (mean 24 hour indomethacin concentration 0.34 micrograms/ml) but in 9 patients (mean 24 hour indomethacin concentration 0.29 micrograms/ml) there was no clinical change. Although the mean 24 hour indomethacin concentration was lower in the group with no clinical change, this was not statistically significant. Five of the 21 patients in whom there was permanent closure of the ductus required more than one dose of indomethacin. The possible effects of birthweight and age at indomethacin treatment were difficult to separate because of the high negative correlation between these two variables. The chance of closure was enhanced significantly if the patient had either a birthweight of at least 1 kg or the age at indomethacin treatment did not exceed 10 days, or both. Six hour but not 24 hour indomethacin concentrations were higher in patients with a high birthweight treated at an early age.