Conventional renal transplantation, which substitutes a single allograft for two native kidneys, imposes an imbalance between nephron supply and the metabolic and excretory demands of the recipient. This discrepancy, which stimulates hyperfunction and hypertrophy of viable allograft nephrons, may be intensified by nephron loss through ischemia-reperfusion injury or acute rejection episodes occurring soon after transplantation. In other settings where less than 50% of the total renal mass remains, progressive glomerular injury develops through mechanisms associated with compensatory nephron hyperfiltration and hypertrophy. To determine whether responses to nephron loss contribute to chronic injury in renal allografts, nephron supply was restored to near-normal levels by transplanting Lewis recipients with two Fisher 344 kidneys (group 2A) compared with the standard single allograft F344 --> LEW rat model of late renal allograft failure (group 1A). At 20 weeks, indices of injury were observed in 1A but not 2A rats. These indices included proteinuria (1A: 45 +/- 13; 2A: 4.0 +/- 0.29 mg/day) and glomerulosclerosis (1A: 23 +/- 4.9%, 2A: 0.7 +/- 0.3%) (p < .05). Double-allograft recipients maintained near normal renal structure and function, whereas 1A rats showed evidence of compensatory hyperfiltration (single-nephron glomerular filtration rate of 63 +/- 10 versus 44 +/- 2.0 nl/min in 2A rats) and hypertrophy (mean glomerular volume of 2.64 +/- 0.15 versus 1.52 +/- 0.05 microns3 x 10(6) in 2A rats) (p < .05). Thus, we conclude that a major component of late allograft injury is attributable to processes associated with inadequate transplanted renal mass, a finding that has major implications for kidney transplantation biology and policy.