Serial krypton 85 curves were recorded in isolated dog kidneys perfused for four hours at 20 to 25 C. Initial curves, recorded at pressures between 80 and 90 mm Hg and flow rates of 70 to 140 ml/min resolved into three rather than the normal four components. When volume flow was kept constant, perfusion pressure rose in four of eight perfused kidneys. Krypton 85 curves recorded at this time showed that almost the entire kidney was perfused at a single rate. Silicone rubber injections of the vasculature of the five nonreimplanted kidneys showed even filling of the entire kidney. In two kidneys, the repeat 85Kr washout curve was unchanged, with no change in the vascular resistance. Neither kidney functioned after reimplantation. Serial 85Kr washout curves were not helpful in assessing the adequacy of hypothermic perfusion techniques.