The reduction of the enormous quantity of data in a radionuclide dynamic study to a few diagnostic parameters presents a problem. Conventional methods of data reduction using regions-of-interest or functional images have several defects which potentially limit their usefulness. Using a principal components analysis of the elemental curves representing the change of activity with time in each pixel, followed by a further factor analysis, it is possible to extract the fundamental functional changes of activity which underly the observed variation of activity. An example of this analysis on a dynamic brain scan suggests that the three fundamental phases of activity represent activity in the arterial system, the venous system and diffusion of tracer into the tissues.