Sodium and water excretion were studied by standard clearance techniques in three experimental models where renal mass was reduced by superficial cortical necrosis (CN) or ischemic segmental infarction (SI). During hydropenia either CN or SI were able to conserve and regulate sodium to a very similar degree. After expansion of extracellular volume, CN reabsorbed less sodium and water than SI. In free-water clearance (CH2O) experiments, the 'apparent distal' sodium delivery was higher in CN than in SI, suggesting a decreased sodium and water reabsorption in the proximal tubules of juxtamedullary nephrons (JM). Both kidneys had similar CH2O when factored for inulin clearance but when CH2O was corrected for 'apparent distal' sodium delivery it was lower in CN than in SI, demonstrating an incapacity of JM to dilute urine. CN also showed less capacity to reabsorb free water than SI. Thus, the use of CN and SI within the same animal was useful to study functional differences between superficial and juxtamedullary nephrons. The present study also suggests that the kidney with superficial CN was unable to perform maximal urine dilution and concentration.