The control of ureteral peristaltic contractions by a pacemaker system is shown in a series of experimental observations on the anesthetized dog. Data are presented to illustrate the influence of pacemaker frequency on ureteral rate and bolus volume during oliguria and transient diuresis. Pacemaker frequency was determined from the pressure wave form of the renal pelvis and peristaltic rate was measured electrophysiologically. The bolus volume associated with each peristaltic contraction was recorded by a drop counter and correlated with pacemaker and ureteral activity. The results show that the pacemaker frequency remains constant over urine flow rates in the range of 0.3 to 15 ml per min. It is also shown that the pacemaker frequency is constant during transient increases in flow rate of more than one order of magnitude. During diuresis, the peristaltic rate changes in quantum steps determined by the fundamental frequency of the pacemaker, and at flow rates greater than 2 ml per min the ureter contracts at the pacemaker rate. Further increases in flow are accommodated by increasing the amount of urine transported by each bolus. The urologic importance of these observations on pacemaker function is discussed in terms of the unicalyceal and multicalyceal upper urinary tract.