Serum concentrations of urea and creatinine, urinary clearances of urea and creatinine, and urine concentrating ability have been proposed as measures for determining renal function in patients with urinary diversion through intestinal segments. Intestinal segments reabsorb urinary solutes, including urea and creatinine, complicating these methods of assessing renal function. This study employs a canine model in which urinary clearances of urea, creatinine and inulin are determined through a normal renal unit and ureter and compared with the contralateral renal unit which has had its ureter replaced by a segment of ileum. Urea, creatinine and inulin are reabsorbed by the ileal segment. Reabsorption of each of these solutes is dependent on urinary flow. The clearance of these solutes through the renal unit with the interposed ileal ureter approaches that of the contralateral renal unit under maximal degrees of diuresis. Creatinine and inulin clearances obtained during diuretic states give the most accurate indication of true renal function. These solutes are reabsorbed to a lesser extent than urea. Diuresis minimizes reabsorption of all these solutes by the ileal segment. Urine concentration does not reflect distal tubule function since the ileal segment reabsorbs urinary solutes and is freely permeable to water.