Although transplanting rat kidneys is an established microsurgical technique, inulin clearance is abnormally low, due to rejection and/or warm ischemia-induced damage. In the present studies, rejection was avoided by using inbred Brown Norway rats as donors and recipients. Donor kidneys were flushed with ice-cold solutions of various composition (saline, saline + 200 or 400 mM mannitol) and pHs (5.7, 6.4, and 7.4), and the kidneys were kept cold during transplantation into unilaterally nephrectomized recipients. Renal function was assessed by clearance techniques 1 week later. In control rats, with both native kidneys intact, the ratio of inulin clearance, left kidney to right kidney, was 0.99 +/- 0.02. In rats with a native right kidney and a transplanted left kidney that had been flushed with saline, the ratio was considerably lower (0.46 +/- 0.09). Adding 200 mM mannitol to the saline flush solution increased the ratio (0.89 +/- 0.09). In comparison, adding 200 mM mannitol and 5 mM phosphate buffer at pH 7.4 resulted in a somewhat lower ratio (0.80 +/- 0.09), whereas adding 200 mM mannitol and 5 mM phosphate buffer at pH 5.7 resulted in a higher ratio, one that was indistinguishable from control (0.97 +/- 0.09). Thus, in this latter group, the inulin clearances of the transplanted kidneys were identical to those of the contralateral native kidneys.