Aim: To study the role of the caliceal system in urine flow. Methods: Eight patients (6 men, 2 women; mean age 42.6 ± 7.6 years) with an upper polar renal mass (6 renal cysts; 2 hypernephromas) which did not encroach on the pelvicaliceal system were studied. After mass removal, a manometric catheter was introduced into one of the intact calices. Another catheter was introduced through the same calix, and the calix was filled with saline in increments of 0.1 ml, and the caliceal, caliceopelvic junction (CPJ), and renal pelvis pressures were recorded. The catheter was moved to the renal pelvis, and the pressure response of the aforementioned sites to renal pelvic filling with saline in increments of 1 ml was registered. The effect of filling of the locally anesthetized calix on the pelvicaliceal pressure was also determined. Results: Caliceal filling resulted in a rise of the caliceal (p < 0.01) and a drop of the CPJ (p < 0.01) pressures with no pressure response in the renal pelvis. Renal pelvic filling produced a rise of renal pelvic (p < 0.01) and CPJ (p < 0.05) pressures; the caliceal pressure showed no change (p > 0.05). Filling of the anesthetized calix or renal pelvis produced no significant pressure changes in calix, CPJ, or renal pelvis (p > 0.05). Conclusions: The CPJ dilatation upon caliceal contraction suggests a reflex relationship which we named ‘caliceopelvic inhibitory reflex’. The reflex is suggested to effect caliceal evacuation. On the other hand, CPJ closure upon renal pelvic contraction postulates another reflex relationship which we designated ‘pelvicaliceal excitatory reflex’. This reflex seems to prevent pelvicaliceal reflux. It is suggested that a ‘functional’ sphincter exists at the CPJ.