The pressure-flow relationship of the normal pyeloureter was investigated in 10 pigs. From a mean baseline pressure in the pelvis of 7.5 cm H2O, the pressure increased during perfusion. The pressure increase from baseline to 8 ml/min was significant. It was noticeable that a perfusion rate of 8 ml/min resulted in intrapelvic pressures of more than 27 cm H2O in 4 of the 10 pigs. A further increase in perfusion rate gave only a slight, additional increase in pressure with linear relationship between flow rate and pressure. The steep rise in pressure from baseline values to 8 ml/min and the slight additional increase in pressure hereafter was probably caused by a transformation of peristalsis to a continuous flow.