The first application of phosphorous 31 (31P) and proton (1H) nuclear magnetic resonance (NMR) spectroscopy to the analysis of the metabolic profiles of skin flaps in a rat model and of human skin grafts is presented. Resonances of adenosine triphosphate (ATP), phosphocreatine (PCr), and inorganic phosphate (Pi) were identified in31P nuclear magnetic resonance spectra. Resonances of phosphocreatine, creatine (Cr), and lactate (Lac) were identified in1H nuclear magnetic resonance spectra. The most significant finding was the substantial presence of phosphocreatine as the major high-energy phosphometabolite in mammalian skin, a finding which heretofore has not been widely recognized. An energy shuttle between phosphocreatine and ATP is operative in skin to buffer the fall in ATP during ischemic (anaerobic) insult. Inability to replenish exhausted phosphocreatine reserves predictively correlates with eventual flap necrosis. We have defined and analyzed temporal fluxes in the phosphocreatine-creatine and phosphocreatine plus creatine-lactate ratios by proton nuclear magnetic resonance. Both are sensitive, accurate, and unambiguous early prognostic indices of eventual flap outcome. These findings support the concept that the fate of a flap may be established as early as 3 hours after elevation and have laid the groundwork for development and application of noninvasive in vivo nuclear magnetic resonance spectroscopy to the study of skin flaps in animals and humans.