Previous studies by others have shown that transplanted rat kidneys have abnormally low clearances of paraaminohippuric acid, inulin, and creatinine, due to rejection and/or to warm-ischemia-induced injury. In the present studies, randomly bred Sprague-Dawley rats were used as donors and recipients. The left kidneys of recipients were removed, and the right kidneys were left intact. Donor kidneys were flushed with an ice-cold hypertonic solution (150 mM NaCl, 200 mM mannitol, pH 6.4), and the kidneys were kept cold during surgery. Renal function was assessed 1 week later. The left transplanted kidneys in untreated recipients exhibited morphologic evidence of rejection, and the clearances of PAH and inulin were approximately 50% of those of the right native kidneys. CsA-treated rats did not reject the transplants, and the PAH and inulin clearances of the left transplanted kidneys were identical to those of the right native kidneys. Untreated and CsA-treated rats with both native kidneys intact served as controls. The amount of CsA given during the 7-day period produced no measurable change in renal function. This is the first demonstration of virtually normal hemodynamics in transplanted rat kidneys when randomly bred animals are used as donors and recipients. Moreover, the results indicate that if both rejection and warm ischemia are avoided, the presence of a functioning native kidney does not have a detrimental effect on the function of a transplanted kidney.