Local deposition density of microspheres is heterogeneous in histologically homogeneous myocardium under physiological conditions. The underlying biological heterogeneity must be distinguished from a methodological heterogeneity which depends preferentially on the number of microspheres injected, blood flow to a particular myocardial region and sample mass. As the variables space (spat), time (temp), and method (meth) are independent of each other, the observed (obs) variability may be approximated using the coefficients of variation (CV) of the individual variables: CVobs = (CV2spat+CV2temp+CV2meth)0.5. Studies in which these different variables have been quantified indicate that the largest fraction of the observed variability of microsphere deposition density is contributed by spatial flow heterogeneity which exists independent of the myocardial layer. Spatial flow heterogeneity increases with decreasing sample mass and decreasing mean flow. Fractal and autocorrelation analyses have shown that adjacent myocardial flows are spatially correlated and nonrandom. Local blood flow was shown to correlate with various metabolic and transport rates, while no differences were found between low and high flow regions with respect to several metabolic markers of tissue hypoxia. In conclusion, the evidence available to date indicates that 1) in histologically homogeneous myocardium there exists a spatial blood flow heterogeneity which 2) is temporally stable, 3) resolution dependent, 4) largely layer-independent, 5) nonrandom, and 6) related to local aerobic metabolism.