The patterns of mechanical contractility in the pyeloureteral pacemaker system were explored in an effort to identify some of the properties of initiating ureteral peristalsis. Rabbit renal pelvises were studied in vivo, in vitro perfused with Krebs solution through the renal artery and in segments. The results show that all segments of renal pelvic smooth muscle are capable of spontaneous contractions, the fornix having the highest frequency with respect to the pelvis. It is also demonstrated that the pelvic contractile wave concentrically accelerates along the renal pelvic perimeter. Under steady state conditions, renal pelvic pressure undergoes a low frequency oscillation having a period of 1--2 min which is flow-rate dependent. The results are discussed within the context of the pyeloureteral pacemaker system and the possible existence of a feedback mechanisms between the renal pelvis and the perfusion of the kidney.