Healthy volunteers (N = 9) and patients with varying degrees of renal insufficiency (N = 36) were given a low dose of dopamine and/or amino acids intravenously during a simultaneous measurement of the glomerular filtration rate and the effective renal plasma flow. Dopamine infusion led to a rise in the glomerular filtration rate and a fall in the filtration fraction. Infusion of amino acids was accompanied by an increase in the glomerular filtration rate while the filtration fraction remained unchanged or increased slightly. The highest values for the glomerular filtration were obtained during the combined infusion of amino acids and dopamine. A reserve in filtration capacity was not or hardly present in patients with moderate (GFR 30 to 90 mliter/min/1.73 M2) to severe (GFR less than 30 mliter/min/1.73 M2) renal insufficiency. We conclude that dopamine decreases total renal vascular resistance while amino acids mainly reduce the tone of afferent arterioles. As amino acids and dopamine seem to be additive in their effects on the glomerular filtration rate, we recommend the combined infusion of these two stimuli to measure renal reserve filtration capacity.