Increased regional myocardial ¹⁸F fluorodeoxyglucose (¹⁸FDG) uptake in relation to ¹³N ammonia (¹³NH₃) uptake - i.e. glucose metabolism-blood flow 'mismatch' - appears to be a strong indicator of myocardial viability in patients with ischemic heart disease (IHD) and regionally reduced contractile function. Reference values of regional ¹⁸FDG and ¹³NH₃ uptake have not been determined in healthy subjects with the target age for the development of IHD. We therefore studied healthy middle-aged and old men using positron emission tomography (PET). Twenty-three healthy men aged 51 to 83 years of age were studied. ¹⁸FDG and ¹³NH₃ uptake was quantified in 16 myocardial segments with PET and circumferential profile analysis. The relative ¹⁸FDG/¹³NH₃ uptake was considerably heterogeneous with ¹⁸FDG uptake consistently higher than ¹³NH₃ uptake in the left lateral ventricular wall. This regional 'mismatch' pattern was observed in all subjects, but was most prominent in middle-aged men. The observed age-dependency was the result of a progressive increased in ¹³NH₃ uptake with advancing age in the left ventricular lateral wall. Age-matched reference values of myocardial ¹⁸FDG and ¹³NH₃ uptake appears to be important for the discrimination between physiological and pathological glucose metabolism-blood flow mismatch assessed by PET and circumferential profile analysis.