A low dose of dopamine was infused in 28 normal volunteers and in 137 patients with varying degrees of renal insufficiency during a routine measurement of the glomerular filtration (GFR) and the effective renal plasma flow (ERPF). Dopamine infusion led to an increase in ERPF and GFR and to fall in the filtration fraction. The effect of dopamine on renal function was most pronounced if the baseline GFR was normal. However, healthy individuals showed greater increases in both ERPF and GFR than renal patients with a comparable baseline GFR. In renal patients no effect was observed if the baseline GFR was below 50 ml/min/1.73 m2. Firstly, it is concluded that already early in renal disease there exists a diminished reserve filtration capacity. Secondly, if the GFR is less than 50 ml/min/1.73 m2, the renal reserve filtration capacity seems to be exhausted.