Using a transparenchymal pressure catheter and a specially designed DC amplifier, rhythmic pressure variations have been demonstrated to occur in the renal pelvis of dogs. For the hydropenic dog this frequency is constant during the course of all the experiments. The interperistalic interval in the ureter constitutes a multiple of the pelvic pressure variations. The ratio of ureteral to pelvic activity is 1 during osmotic diuresis. The frequency of the ureteral activity was never found to be larger than the pelvic activity. These experiments suggest that the ureteral contractions are correlated to the pelvic activity.