Partial obstruction of the left ureter was created in 2-day-old rats and released 2 days later. The effects were studied after 1, 2, 3 or 6 weeks. The obstruction resulted in prominent hydronephrosis, but the pelvic volume returned to normal already within 1 week after the release operation. In most of the previously obstructed kidneys there were small regions with moderately widened convoluted tubules and collecting ducts and minor areas with mild chronic pyelonephritis; these lesions were similar after 1 week as after 2-6 weeks, indicating that they were permanent. Judging from the parenchymal weight difference between the right and left kidney, there was slight reduction on the previously obstructed side at the 3rd week, balanced by hypertrophy on the contralateral side. At the 6th week the parenchymal weight reduction was no longer apparent but there was still evidence of contralateral hypertrophy. Thus, early release of the partial ureteric obstruction prevented persistent ipsilateral parenchymal weight reduction, without ensuring healing of the tissue lesions. The catch-up of parenchymal weight is probably due to the fact that the release was performed before the end of the period of postnatal nephron differentiation. Interference with this period seems to be the main cause of parenchymal weight reduction in permanent neonatal partial ureteric obstruction.