Renal function in 12 live kidney donors who had donated a kidney to a relative 3 to 10 years previously was studied. No clinically significant impairment of renal function was observed in the group. A significant rise in creatinine clearance (P = less than 0.01) occurred after a meat-protein load. When compared with 12 healthy controls with two kidneys, there was no significant difference in the percentage change in clearance after the meat load. In both groups there was an inverse correlation between the percentage change in creatinine clearance and the baseline creatinine clearance (P = less than 0.01), with no significant difference in the regression lines of the two groups. In conclusion, it appears that the single kidney responds appropriately to a meat-protein load and that there is no evidence from this study to suggest that hyperfiltration damaged the remaining kidney.