A partial obstruction of the left ureter was created in six-week-old rats. The effects on renal function were studied after three, nine and 15 weeks, first in normal hydration, and then after extracellular volume expansion. Moderate hydronephrosis without parenchymal weight reduction developed within three weeks. The hydronephrotic kidney i) excreted during normal hydration less urine and sodium than the intact one, because of increased reabsorption, ii) was capable of reacting fully on volume expansion and iii) had, after volume expansion, a higher renal blood flow and GFR but also a higher reabsorption of water, sodium, potassium and osmoles, resulting in excretions similar to those on the intact side. The differences noted were small (less than 20%) except for sodium excretion. The hydronephrotic kidney seemed to tolerate an increase in ureteral resistance better than the intact one would do. There were no significant differences between the three, nine and 15-week groups, with regard to the effects on the hydronephrotic kidney. Thus, except for a tendency to sodium retention, the effects of partial ureteric obstruction in young rats seem to be relatively harmless and do not increase with time.