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Unfortunately, the nonoperative treatment of hydronephrosis, in wide use today, cannot be evaluated until after many decades. Long-term animal experiments offer a peekhole into the future. Partial obstruction of the left ureter was created in newborn rats. Its effects were studied after 1 year. In order to stress the system, the investigations were performed under hydropenic conditions. On the obstructed side, the renal pelvis was considerably enlarged; however, the microstructure of the kidney was only slightly affected. Renal blood flow was decreased (-17%) due to a rise in vascular resistance. Filtration was less affected (-9%), probably as a result of increased filtration fraction. Water excretion, including that of free water, was increased (+59%) due to a decreased reabsorption. Potassium excretion was decreased (-59%) either due to increased reabsorption and/or decreased secretion. Sodium excretion followed a similar pattern (-24%) although without statistical significance. The contralateral side compensated for the ipsilateral decreases in blood flow and filtration. Ipsilateral tubular functions were not compensated for, which indicates an acute situation that is reaffirmed by the finding of a normal hematocrit and serum osmolality/electrolytes. The changes in tubular functions were thus considered to be due to the hydropenic stress, but inescapably an incompetent concentration capacity exists. The changes in the obstructed kidney were thus moderate, some of which were fully compensated for, but, most important, all were nonprogressive. The study supports the nonoperative treatment of these patients, provided that the follow-up guarantees catch-up of the exceptional cases that deteriorate, especially during infancy.