Acute renal failure has frequently been reported following operations which require cross-clamping the abdominal aorta below the renal arteries. In the past 10 years, incidences of up to 47% decreased renal function, 37% oliguria, and 11% mortality have been reported [30]. Although modern developments in patient care, anesthesia, and operative technique have reduced the incidence of renal failure to 3-5% [20, 401, the mortality of this complication approaches 25% despite aggressive therapy [ 11. Recent measures to prevent this sequela have focused on adequate replacement of crystalloid during operation. Using this technique, Thompson et al. reported a mortality from renal failure of only 0.2% in 673 cases of aortoiliac surgery [41]. Despite these improvements, the underlying etiology of renal failure following aortic surgery remains obscure. Early animal experiments demonstrated acute tubular necrosis, which was attributed to aortic crossclamping, per se, and was reversed by direct or pharmacologic renal nerve blockade [27,31]. Subsequent investigators, however, were unable to substantiate these findings [5, 17, 20, 221, and it was generally concluded that aortic cross-clamping has no detrimental renal effect separate from laparotomy alone [9, 151. Recently, however, reports by Abbott and Berkowitz and their colleagues have contested this conclusion [l, 21. Using the 133Xe washout technique to measure the distribution of intrarenal blood flow, these investigators demonstrated decreased cortical blood flow with increased juxtamedullary and outer medullary flow during aortic crossclamping. They suggested that this technique detects more subtle renal damage than previously appreciated by other measurements and that, in fact, aortic crossclamping does contribute to renal complications. Recently, microsphere techniques have been introduced to quantitate the distribution of intrarenal blood flow directly. Several investigators have compared the microsphere and xenon washout techniques and have reported discrepancies, especially under abnormal physiologic conditions [23, 35, 37, 391. Criticism of the xenon technique prompted the present study, which was designed to analyze the distribution of intrarenal blood flow during aortic clamping and declamping using a direct microsphere technique. The results of this study do not support previously reported shifts of intrarenal blood flow measured by xenon-133 washout.