The prostaglandin synthetase inhibitors diclofenac and indomethacin relieve pain in renal colic probably by decreasing renal pelvic pressure. Decreased diuresis and effects on the oedema around the obstructing stone are plausible explanations. In the present study on rabbit renal pelvic tissue strips it was found that both indomethacin and diclofenac decreased the spontaneous phasic activity of the strips. ED50 was 5.4 +/- 0.7 X 10(-5) M for indomethacin and 2.4 +/- 0.5 X 10(-5) M for diclofenac (P less than 0.001). With the prostaglandin synthetase inhibitors still in the tissue bath the activity was regained when either prostaglandin E2 or F2 alpha was added, prostaglandin E2 being more potent in this respect. It was concluded that prostaglandin synthetase inhibitors seem to reduce smooth muscle activity in the renal pelvis, a mechanism which might contribute to the pain-relieving ability of these drugs in renal colic.