In a multicentre study the analgetic effect of indomethacin in ureteral colic was analyzed. Indomethacin, which inhibits the synthesis of prostaglandin, was given by intravenous infusion in a dose of 50 mg. The study comprised 198 patients with 252 attacks of ureteral colic. Satisfactory relief of pain was obtained in 212 attacks (84%). No serious side effects of indomethacin were observed. The pain recurred within four hours in only 15 patients (7%), in 12 of them during a diagnostic intravenous pyelography. The relief of pain may be explained by reduced glomerular filtration pressure and/or diminished urinary excretion, causing rapid fall in the renal pelvic pressure and in the tension of the pelvi-ureteral wall. A low extracellular volume of fluid in a patient with ureteral colic may be essential for analgetic action of indomethacin.