To obtain functional images of brain perfusion, a new concept was developed which takes advantage of the short half-life of a radioactive tracer. Under continuous intracarotid infusion of a solution of 81mKr (T1/2 [half-life] = 13 s; produced from its parent, 4.6 h 81Rb), this tracer never reaches equilibrium within the dog brain because of the rapid radioactive decay. Its distribution reflects regional arrival of the nuclide, indicating regional cerebral blood flow rather than volume. During continuous infusion of 81mKr, perfusion images can be obtained by simply collecting counts with a .gamma.-camera and recording on Polaroid film. The procedure is readily repeatable to get images in multiple views or to follow minute-by-minute changes of cerebral perfusion.