The effect of polycythemia on the coronary microcirculation was studied in young male rats. Two experimental models of polycythemia were employed: cobalt-induced polycythemia, which mimics hypoxia-induced changes, and erythropoietin-induced polycythemia, which circumvents these changes. In both models, baseline left ventricular function was normal, whereas maximal systolic and developed pressures were decreased. In cobalt-treated rats the left ventricular functional reserve was also compromised. Morphometric analysis of the left ventricle confirmed previously described improved geometric conditions for oxygen supply at the distal portions of capillaries (smaller domain areas and shorter capillary segments). In cobalt-treated but not in erythropoietin-treated rats, increased capillary angiogenesis was also detected. In the hearts from rats with both types of polycythemia, a small but significant increase in the formation of arterioles was found. Capillary linear hematocrit was within the normal range in both types of polycythemia despite sizeable increases in systemic hematocrit. Significant differences in red blood cell distribution within capillaries were found between proximal and distal portions in all experimental groups.