Cardiac output and regional blood flow distribution were measured in 14 newborn dogs before and 90 min following 0.3 mg/kg of indomethacin and in 4 control animals who received buffer alone using the Radioactive Microsphere Reference Organ Technique. Indomethacin produced no significant change in cardiac output or blood flow to the gastrointestinal tract or kidney. There were no changes in cerebral blood flow in animals over 4 days of age. However, in 3 of 8 puppies less than 3 days of age, indomethacin resulted in a 47% fall in cerebral blood flow. In newborn dogs, indomethacin in the dose employed had no deleterious effects on cardiac output and, with the possible exception of the cerebral circulation, on blood flow distribution.