Congenital obstructive nephropathy is the most important cause of renal insufficiency in infants. Over 90% of nephrogenesis in the rat takes place in the first 10 d postnatally, analogous to the midtrimester human fetus. To determine the renal consequences of temporary UUO during the period of nephrogenesis, the left ureter of 1-2 day-old rats was sham-operated (S) or occluded (UUO) and animals were studied 5 d later (n=14). Compared to S, UUO reduced the left kidney glomerular maturation index (1.33 +/- 0.05 vs. 1.62+/- 0.05), reduced glomerular number by 40%, and increased the fraction of atrophic tubules 18-fold (all p<0.05). In additional rats, UUO was released 5 d after occlusion (Rel) or not released (XRel), and the left kidney was removed 28 d later (n=32). Compared to S, glomerular number in Rel was reduced 40% (p<0.05), while glomerular area was not different. Compared to S, renal tubular cell immunoreactive transforming growth factor beta-1 was increased 3-fold in both XRel and Rel (p<0.05); epidermal growth factor was reduced 75% in XRel vs. 20% in Rel (p<0.05); vimentin was increased 24-fold in XRel and 6-fold in Rel (p<0.05); clusterin was increased 4-fold in XRel and not in Rel (p<0.05); and tubular atrophy was increased >100-fold in XRel and 12-fold in Rel (p<0.05). Left kidney glomerular filtration rate (GFR) did not differ between S and Rel rats 28 d after operation (0.32 +/- 0.07 vs. 0.35+/- 0.05 ml/min/g KW, respectively) (n=11). In summary, 5 d UUO in the neonatal rat impaired glomerular maturation and reduced glomerular number, which persisted despite release of UUO. Renal tubular growth factor expression remained abnormal and tubular injury (evidenced by vimentin and clusterin expression) and tubular atrophy persisted at 28 d despite release of UUO. Remarkably, GFR of the postobstructed kidney remained normal in the face of a marked loss of nephrons and tubular abnormalities. We conclude that even temporary urinary tract obstruction during the period of nephrogenesis irreversibly impairs renal development, which may not be reflected by a reduction in GFR in the neonatal period. The functional consequences of temporary urinary tract obstruction in early development may therefore not be manifested until adulthood.