Renal functional reserve represents the capacity of the kidney to increase its level of operation in response to certain demands. The reserve of glomerular filtration rate and of renal blood flow is discussed from the following points of view: evaluation, measurement, mechanisms involved and significance. Data from the literature are discussed which show (i) that the mechanism of the hyperfiltration seen in the early stage of diabetic nephropathy may be different from the hyperfiltration induced by infusion of amino acids, (ii) that the remaining kidney in healthy kidney donors maintains its functional reserve, and (iii) that the functional reserve is fairly well maintained as long as the glomerular filtration rate is decreased only moderately. The reserve of tubular functional capacity is discussed from the point of view of concentration and dilution and of acidification and alkalinization.