The effects of indomethacin on furosemide induced changes in renal blood flow were determined in dogs. Furosemide alone caused an increase in total renal blood flow while indomethacin alone decreased renal blood flow. When furosemide was administered to animals pretreated with indomethacin the increase in renal blood flow was blocked. Changes in intrarenal blood flow distribution were also measured using radioactive microspheres. The pattern of blood flow distribution after furosemide was modified in some of the animals pretreated with indomethacin. Stimulation of renin secretion occurred after furosemide in indomethacin-treated animals. The data suggest that the changes in renal blood flow produced by furosemide may be modulated by the prostaglandin system.