The term "renal functional reserve" (RFR) refers commonly to the reserve of glomerular filtration rate (GFR) and renal blood flow. RFR can be elicited by an oral protein load or by infusion of aminoacids, glucagon, or dopamine. The increase in GFR which follows aminoacid administration results from a cascade of events including at least pancreatic release of glucagon, involvement of an hepatic step yet unidentified, and renal synthesis of vasodilatory prostaglandins. RFR represents a constant fraction of baseline GRF as long as the latter is above 40-50 l/min. It has been suggested tha permanent challenge of RFR, which occurs in protein-rich diet or during the hyperfiltration phase of diabetic nephropathy, might lead to and accelerate impairment of renal function. The relevance of RFR measurement as a tool to predict the evolution of renal function in various types of renal diseases remains to be evaluated.