A partial obstruction of 1 ureter was created in newborn rats and its effects were studied in the adult rat. The obstructed pelvis was found to be considerably enlarged. Nevertheless, the GFR (glomerular filtration rate) was only slightly decreased (10 per cent), completely compensated by increase on the contralateral, non-obstructed side. The reduction in GFR was less than the reduction in number of glomeruli (19 per cent), indicating a raised filtration rate per glomerulus. Water excretion was slightly increased and potassium excretion moderately decreased; sodium and osmolar excretion were not significantly affected. There was no correlation between these changes and the degree of pelvic enlargement. Thus, in this model, in which there is no urinary tract infection or pyelonephritis, partial obstructive uropathy caused less damage to the kidney function than might have been expected.