Many aberrations of ureteral drainage were observed during cinefluorographic studies of 43 subjects with partial or complete duplication of the renal collecting system. In many cases only 1 of the 2 ureters was seen to have peristaltic contraction in any position. Frequently only "wiggling" peristaltic movements were visualized, and often the uretero-pelvic junction of the upper ureter was high, near the infundibulum. An outstanding finding was the occurrence of retrograde ureteral peristalsis in 84% of the patients with partial duplication or bifid ureter. This occurred when an antegrade peristaltic wave from 1 limb arrived at the junction and flooded contrast medium back to the pelvis and calyceal system of the other limb. It is thought that this occurs throughout the patient''s lifetime and that it is of no clinical significance except in the event of pathogenic infection, when serious and chronic infection or even septicemia may result. The findings obtained during this study of renal dynamics indicate that simple uretero-ureteral anastomosis high near the renal pelvis would constitute a logical procedure in the treatment of bifid ureter with chronic uncontrollable infection.