It is presumed that a region of myocardium does not always have the same blood flow. The purpose of this communication is to examine those factors which are determinants for changing regional flow. We have divided the determinants into three types. Those affecting a regional variation in blood flow which has a random distribution and a cyclic character which will be termed spatial and temporal heterogeneity (1). The second group are the determinants of the transmural distribution of coronary blood flow which were described by Griggs et al (2), Buckberg et al (3) and modified by L’Abbate et al (4). Finally, there are those determinants which are present when coronary artery obstruction occurs and collaterals develop which redistribute myocardial blood flow in a geographical sense (5,6).