To define the pattern of recovery from postischemic acute renal failure (ARF), we performed clearance and micropuncture studies at intervals of 1, 2, 4, and 8 weeks following 60 min of complete unilateral renal artery occlusion in the rat. At 1 week, the inulin clearance (CIn) of the postischemic kidney was less than 2% of normal. The presence of marked preglomerular vasoconstriction was indicated by the reductions in renal blood flow (RBF), and stop-flow (SFP) and estimated glomerular capillary hydrostatic pressures (GCPe). In additon, there was evidence of tubular obstruction. Proximal intratubular pressures (PITP) were elevated, and intratubular casts could be seen in vivo and on histologic sections. At 2 weeks CIn had increased more than tenfold. This change occurred in the absence of any significant elevation in RBF, SFP, or GCPe. PITP had fallen, however, to normal values, and histologic sections revealed a marked reduction in the extent of intratubular casts. Ipsilateral urinary recovery of 3H-inulin microinjected into proximal convolutions was complete. At 4 and 8 weeks, there were further but more gradual rises in CIn, which were associated with progressive increases in RBF, SFP, and GCPe. These observations indicate that recovery from postischemic ARF occurred in a biphasic pattern. The initial rise in CIn was associated with the relief of intratubular obstruction, whereas subsequent rises in CIn occurred in association with progressive renal vasodilation.