The recent introduction of newer scintillation cameras, convenient radiopharmaceuticals and innovative data-processing systems has hastened the use of radionuclide methods in the evaluation of cardiac function. This review familiarizes the primary internist with physics and instrumentation required in understanding cardiovascular nuclear images. Basic principles underlying the gated cardiac blood-pool scan and the first-pass study are described. Authoritative references are given to readers seeking more complete information. The major emphasis of the review, however, is on the clinical applications of these techniques. The role of these noninvasive methods in myocardial infarction, diagnosis and prognosis of ischemic heart disease, assessment of cardiomyopathies, study of idiopathic hypertrophic subaortic stenosis and left atrial myxoma is discussed. Finally, mention is made of newer applications: evaluation of nitroglycerin action on viable but ischemic myocardium and determination of the hemodynamic effects of propranolol in patients with coronary-artery disease.