Fluorescence emission of reduced nicotinamide adenine dinucleotide (NADH) from the surface of perfused rat hearts was photographed to provide a two-dimensional recording of NADH levels. Sodium Amytal inhibition of NADH oxidation resulted in a homogeneous increase in NADH fluorescence, while lowering perfusion pressure from 55 to 10 torr caused a heterogeneous increase in NADH fluorescence, reflecting the heterogeneous oxygen delivery at this low pressure. Local ischemia resulted in a well-defined region of high NADH fluorescence that corresponded to the region of ischemic inslut. The sharp transition between the ischemic and normoxic areas demonstrated that the hypoxic interface separating the two areas must be quite small.