Background: It is reported that G-CSF and Erythropoietin (Epo) enhanced angiogenesis in ischemic myocardium. However, the effect to collateral formation (arteriogenesis) was not fully investigated. Method: Male Wistar rats (6 wks, n=24) were used. After ligation of left anterior descending coronary artery, rats were divided into three groups (each n=8): G-CSF (10 ug/kg/day, 4 days, S.C.), Epo (1,000 IU/kg, single use, I.P.) and Control (C). 4 weeks after ligation, coronary angiography was performed in Langendorff isolated rat heart by synchrotron radiation angiography (SRA). SRA can identify down to 50 um of coronary arteries. Collateral formation was assessed by (1) number of collateral branches towards ischemic area from right coronary artery (RCA) and (2) total length of collateral arteries from RCA. All images were taken in frontal view of coronary arteries. A computer software “NIH image” was employed for measurement of collateral length. Result: Coronary ligation and myocardial infarction was equivalent in all cases. Number of branches were 8.1 +/− 0.8 (G-CSF), 7.5+/−1.4 (Epo), and 6.3+/−1.4 (C) (p<0.01 G-CSF vs C). Total length of collateral arteries were 46.1 +/− 7.0 mm (G-CSF), 42.4 +/− 11.9 mm (Epo), and 27.5 +/− 11.0 mm (C) (p<0.005 in G-CSF vs C, p<0.05 in Epo vs C). Conclusion: Both G-CSF and Epo enhanced collateral formation compared with untreated myocardial infarction in addition to angiogenesis.