[en] The functions of the kidneys and the distribution of those functions are best investigated by 'physiological' tests, in which the use of radionuclides predominates. There are basically only two functions that can be measured with radionuclides: the uptake function, the ability of the kidney to take up any substance from the blood whether through the glomerulus or tubule; and the transit function, the time of movement of substances from uptake point to output. The distributions of these functions may also be imaged or analysed. From these two basic measurements the others such as the rate of output of substances from the kidney may be derived. The methods used include probe renography, static and dynamic imaging with the computer linked camera and the in vitro measurement of blood or urine samples. Each of these methods is described in detail. (Auth.)