Regeneration of the ureter and renal pelvis from a transected and anastomosed strip of the ureteral wall was examined in 20 dogs. The latter were divided into two groups: group A, dogs operated without receiving any drug, and group B, dogs which received 1.5 mg prednisone and 1.5 azathioprine/kg body weight/day before the operation and throughout the postoperative period. Provided the ureteral splint was maintained for at least 28 days, good results were obtained in both groups. Long-term administration of the drugs thus had no adverse effects on the regeneration. Microscopic examination of the newly formed wall, showed more numerous and better arranged muscular fibers and less connective tissues in group B.