Affected rats of the unilateral urogenital anomalies (UUA) strain show renal agenesis restricted to the left side. To determine whether unilateral renal agenesis is a risk factor for the progression of renal insufficiency, we studied age-related pathophysiological alterations in affected rats. Although body growth and food intake were normal, polydipsia and polyuria with low specific gravity were present at 10 weeks and deteriorated further with age. Blood hemoglobin concentrations were normal, though there was slight erythropenia with increased MCV and MCH. Although hypoalbuminemia, hypercholesterolemia, azotemia, and hypermagnesemia were manifested after age 20 weeks, neither hyperphosphatemia nor hypocalcemia was observed. Plasma Cre and UN concentrations gradually increased with age. Cre clearance was almost normal, whereas fractional UN excretion was consistently lower than normal. Proteinuria increased with age, and albumin was the major leakage protein. In addition to cortical lesions, dilated tubules, cast formation, and interstitial fibrosis were observed in the renal medulla of 50 week-old affected rats. Renal weight was increased 1.7-fold and glomerular number 1.2-fold compared with normal rats. These findings show that the remaining kidney in UUA rats is involved not only in compensatory reactions but experiences pathophysiological alterations associated with progressive renal insufficiency.