Although obstructive nephropathy is a major cause of renal insufficiency at all ages, the functional consequences of relief of obstruction on the developing kidney are poorly understood. To evaluate recovery from chronic partial ureteral obstruction (CPUO) in the neonatal period, the left ureter of guinea pigs was constricted within the first 48 hours of life, and the obstruction was relieved 10 days later. At three and eight weeks of age, intraureteral pressure, number of perfused glomeruli (NPG), renal blood flow (RBF), and glomerular filtration rate (GFR) were measured. These animals were compared with sham-operated and unrelieved groups. Two additional groups underwent CPUO at five weeks of age, with persistent obstruction or relief in 10 days, and were also studied at eight weeks. In all animals, intraureteral pressure increased during ipsilateral ureteral obstruction and normalized following its release. Recovery of RBF and GFR after relief of ipsilateral CPUO in the newborns was only partial at eight weeks, with no renal growth or increase in NPG from three to eight weeks of age. In contrast, recovery of RBF in the adult was complete 10 days after relief of obstruction, with significant increase in GFR, and no decrease in renal mass or NPG. We conclude that if delayed, relief of CPUO in the neonate may not restore renal growth and functional maturation which have been impaired by CPUO.