Obstruction of the urinary tract is associated with an increase in pelvic pressure (PP) and a decline in ipsilateral renal blood flow (RBF). To investigate the influence of the renal prostaglandins on these parameters, pigs with complete unilateral ureteral obstruction (UUO) were studied under general anesthesia after administration of indomethacin 30 minutes before obstruction. Pelvic pressure increased to a maximum of 41.1 +/- 2.4 cm. H2O after 6 hours, but maximum pressure, as well as the rate of pressure increase, was significantly reduced after indomethacin. A transient but significant increase in mean aortic blood pressure was seen, together with a bilateral decrease in RBF immediately after indomethacin administration. Ipsilaterally RBF decreased by 25% from 319 +/- 21 ml. per minute to 237 +/- 16 ml. per minute and contralaterally by 23% from 300 +/- 20 ml. per minute to 231 +/- 25 ml. per minute. After 15 hours a further decline was measured in ipsilateral RBF to 136 +/- 20 ml. per minute but, on the contralateral side, RBF was only slightly reduced to 281 +/- 36 ml. per minute after 15 hours. Following indomethacin a sustained increase in ipsilateral renal vascular resistance (RVR) was observed from 37.6 +/- 3.6 mm. Hg x ml.-1 x min. x gm. to 130.5 +/- 42.3 mm. Hg x ml.-1 x min. x gm. compared with a much smaller increase from 37.4 +/- 3.2 mm. Hg x ml.-1 x min. x gm. to 55.1 +/- 9.2 mm. Hg x ml.-1 x min. x gm. in the control group. Renal secretion rate of angiotensin II was significantly reduced 2 and 6 hours after indomethacin administration on the obstructed side, and was not significantly changed on the contralateral side. It can be concluded that inhibition of prostaglandin synthesis during unilateral ureteral obstruction in pigs results in reduced pelvic pressure, renal blood flow and angiotensin II secretion rate from the affected kidney, and that renal prostaglandin synthesis plays an important role for the perfusion of the kidney during ureteral obstruction.