Sulfur heteroatom substitution at the C4 position of .sup.18 F-labeled fatty acids yields a tracer that is retained in proportion to the beta-oxidation rates in mammalian tissue, particularly hypoxic myocardium. Most preferably, the invention is embodied in an [.sup.18 F]fluoro-4-thia-fatty acid having a chain length of between 8 to 20 carbon atoms. The .sup.18 F-labeled 4-thia fatty acids of this invention find particular utility the radiolabelling of tissue sites (e.g., myocardium) for purposes of positron emission tomography.