The aims of this study were to determine the effects of dopamine and a low protein diet on glomerular filtration rate and effective renal plasma flow in the aged kidney. Effective renal plasma flow was measured using 125I-labelled hippuran and glomerular filtration rate using 51Cr-labelled EDTA. Low-dose continuous intravenous dopamine 3 micrograms.kg-1.min-1 in 10 healthy elderly volunteers caused a significant increase in effective renal plasma flow but not in the mean glomerular filtration rate when compared with baseline. However, glomerular filtration rate did increase substantially in 5 subjects (mean 14.4, SD 1.3). This implied that the elderly kidney was working maximally without reserve capacity in half the elderly. Since renal function is likely to be even more reduced in elderly patients with congestive cardiac failure, dopamine infusions may have little place in this condition in some older patients. A low protein diet (0.69 g.kg-1) in the same volunteers reduced glomerular filtration rate, suggesting that protein restriction may help to reduce the increased filtration rate in the remaining nephrons, thereby leading to structural and functional preservation in the aged kidney.