Regional myocardial flow distributions in Langendorff rat hearts under Tyrode and blood perfusion were assessed by digitalradiography (100-µm resolution) using tritiated desmethylimipramine as a molecular flow tracer. Spatial patterns of flow distribution during baseline and maximal hyperemia following a 60-sec flow cessation were evaluated by the coefficient of variation of regional flows (CV, related to global flow heterogeneity) and the correlation between adjacent regional flows (CA, inversely related to local flow randomness). These values were obtained for the original images (64² pixels) and for coarse-grained images (32², 16², and 8² blocks of nearby pixels, i.e., pixel aggregates). At a given point in time during baseline, both CV and CA were higher in blood (n=7) than in Tyrode perfusion (n=7) over all pixel aggregates (P<0.05, two-way ANOVA). During the maximal hyperemia, CV and CA were still significantly higher in blood (n=7) than in Tyrode perfusion (n=7); however, these values decreased substantially in blood perfusion and the CV- and CA-differences became smaller than those at baseline accordingly. During basal blood perfusion, the 60-sec averaged flow distribution (n=7) showed the smaller CV and CA than those at a given point in time (P<0.05, two-way ANOVA). Coronary flow reserve was significantly higher in blood than in Tyrode perfusion. In conclusion, the flow heterogeneity and the local flow similarity are both higher in blood than in Tyrode perfusion, probably due to both the different degree of coronary tone preservation and the presence or absence of blood corpuscles. Under blood perfusion, temporal flow fluctuations for short-term periods of 60-sec order are largely involved in shaping microregional flow distributions.