The remaining kidney undergoes compensatory hypertrophy after donor nephrectomy. Studies have shown that immediate increases of renal plasma flow (RPF) and glomerular filtration rate (GFR) take place. No studies of tubular functions in kidney donors have previously been published. Nine elderly kidney donors with a mean age of 55 years (range 48-71 years) underwent the following studies before and 6 and 12 months after nephrectomy: GFR, RPF, maximal tubular secretion of para-aminohippurate (TmPAH), net acid excretion after 3 days of ammonium chloride loading and test of the urinary concentration ability. RPFP and GFR decreased postoperatively to a mean of 61 and 63.5 per cent respectively, with a rise of the filtration fraction (FF). TmPAH decreased considerably less, to 76 per cent 6 months postoperatively and 86 per cent 12 months postoperatively. The TmPAH/GFR ratio changed from 0.77 to 0.95 and 0.99 postoperatively. The excretion of acid was slightly reduced concerning the NH4+-excretion but the values for H+-excretion and urinary pH were unchanged. The remaining kidney after donor nephrectomy shows the well-known increases in RPF and GFR. Energy-dependent proximal tubular functions such as TmPAH increase even more, indicating an altered glomerulotubular balance. Distal tubular function such as acidification and urinary concentration ability are essentially unchanged.