[en] Radionuclide methods provide a capability to visualize regional function in living human organ by administration of extremely small amounts of radioactive tracers. Both position emission tomography (PET) and single-photon emission computed tomography (SPECT) were introduced in medical research in the late 1970's, and have been extended to clinical practice over the past decade. PET offers a unique capability to measure both physiological and biochemical processes by taking the advantage of the use of short-lived positron emitting radionuclides, such as C-11, N-13, O-15, and F-18. SPECT is currently used widely in the clinical setting due to the commercial delivery of labeled tracers and the development of instruments that allow imaging in various views. This article reviews the application of PET and SPECT in the diagnosis of brain and heart diseases, focusing on function, blood flow, and metabolism of these organs. (N.K.)