The presence of delayed graft function (DGF) following cadaver donor renal transplantation is associated with inferior graft survival as well as decreased patient survival. Delay in onset of function eliminates a valuable indicator of allograft viability, which is not easily replaced by standard diagnostic procedures. The purpose of this study was to demonstrate that a new clearance technique could be used to measure renal function minute to minute and under conditions similar to those observed in humans in the immediate posttransplantation period. A monkey model was used to provide controlled conditions. Increasing levels of ischemic injury were produced in 12 Rhesus monkeys by renal hilum cross-clamping. Real-time measurements of glomerular filtration rate (GFR) were obtained from the rate of clearance of the extracellular fluid of the GFR agent99mTc-DTPA, as measured with a specially designed external radioactivity counting device called the ambulatory renal monitor, or ARM. GFR was measured every 2–5 min as the slope (k) of the log of activity measured minute to minute versus time. GFR measurements were correlated with blood urea nitrogen (BUN), plasma creatinine (Cr), routine light microscopy, and measurement of proliferating cell nuclear antigen (PCNA), a marker of cell proliferation. Large changes in renal function due to ischemia or ureteral obstruction were observed within minutes. In addition, the rate constant on Day 1 was predictive of peak serum Cr (R = -0.86, R2=. 74, p =. 0001). Acute tubular necrosis (ATN) resolution was reflected more quickly when using the rate constant (Day 1) than when using either BUN or plasma Cr (Day 3–4). Because of renal functional reserve, BUN and plasma Cr were relatively insensitive indicators of mild to moderate reductions in GFR as compared with the rate constant. We conclude that ARM is a simple method which provides an accurate, near real-time GFR readout with potential applications not only for the clinical management of patients with DGF, but also as a research tool in acute renal failure (ARF).