Recovery from unilateral postischemic acute renal failure produced by 1 hour of complete renal artery occlusion in the rat is incomplete and is associated with the eventual loss of renal mass. The loss in renal mass can be reversed by removal of the contralateral kidney. In the present study, clearance and micropuncture techniques were used to compare whole kidney and individual nephron function of normal and postischemic kidneys. In sham-operated control animals, the left kidney weight (LKW) averaged 0.41 g/100 g body weight (BW), the inulin clearance (Cin) averaged 541 μl/min per 100 g BW and single nephron glomerular filtration rate (SNGFR) averaged 15.4 nl/min per 100 g BW. The calculated number of nephrons, derived from the relationship Cin/SNGFR, was 37,780. Four weeks after the period of ischemia, the LKW of rats with the contralateral kidney in place averaged 0.30 g/100 g BW. Cinaveraged 158 μl/min per 100 g BW and SNGFR averaged 17.4 nl/min per 100 g BW. The calculated number of nephrons was 9460. In rats in which either the contralateral kidney had been removed or its ureter ligated 2 weeks following the period of ischemia (2 weeks prior to study) LKW averaged 0.66 g/100 g BW, Cinaveraged 415 μl/min per 100 g BW, and SNGFR averaged 19.4 nl/min per 100 g BW. The calculated number of nephrons was 24,152. The results indicate that the increases in size and Cinof the postischemic kidney which occurred in response to the contralateral nephrectomy or ureteral ligation were due to the functional recruitment of nephrons which otherwise would have become atrophic. These data also demonstrate that the extent of functional recovery from post- ischemic acute renal failure is not necessarily dependent on the nature of severity of the initial insult. Circ Res 46: 440-448, 1980