Fetal uninephrectomy (uni-x) results in hypertension at a later age in female than male sheep. We hypothesized that dysregulation of tubular sodium handling contributes to the reduced ability to regulate extracellular fluid (ECF) homeostasis in older females born with a congenital nephron deficit. Following renal excretory balance studies, the response to inhibition of the Na(+)K(+)2Cl(-) cotransporter with furosemide (0.5 mg/kg bolus + 1 mg/kg per hour, i.v) or vehicle treatment was examined in conscious 5-year-old female uni-x (n = 7) and sham (n = 7) sheep. Balance studies in meal-fed sheep demonstrated that while average 24 h sodium excretion over 6 days was not different between the groups, the daily variation in sodium excretion was significantly greater in uni-x compared to sham sheep (31 +/- 4 vs. 12 +/- 2 ; P <0.001). Basal plasma renin activity (PRA) and renal cortical cyclooxygenase-2 (COX-2) gene expression were lower in uni-x sheep (both, P <0.01). The increases in glomerular filtration rate (GFR) and renal blood flow observed in sham sheep in response to furosemide were significantly attenuated in uni-x sheep (both P GROUPxTREAT <0.05). However, fractional sodium excretion increased by a greater extent in the uni-x (4.4 +/- 1.0 ) as compared to the sham sheep (2.0 +/- 0.4 ; P GROUPxTIME <0.05) in response to furosemide. In conclusion, fetal uni-x was associated with altered renal sodium handling and hypertension in aged females. The impaired ability to modulate PRA and GFR in the adults with a congenital nephron deficit may reduce the capacity of the kidney to respond to gains or losses in ECF to maintain a stable internal environment.