We visualized the epicardial capillary network of the beating canine heart in vivo, using our high resolution intravital CCD videomicroscopy system, in order to elucidate its functional role under control conditions and during reactive hyperemia (RH).We used10hearts of open chest, anesthetized dogs.The pencil-lens videomicroscope probe was placed over capillaries fed by the left anterior descending artery in A-V blocked hearts paced at 60-90 beats per minute.In individual capillaries under control conditions flow was predominant either during systole or during diastole, indicating that the watershed between diastolic arterial and systolic venous flows is located within capillaries.The capillary flow increased rapidly during RH and reached a peak flow-velocity (2.0±0.2mm/s), twice as high as the control flow-velocity (1.0±0.3mm/s), with enhancement of intercapillary cross-connection flow.Furthermore, the mean diameter increased by15%compared with the control value, additionally facilitating oxygen supply to myocardial cells, but there was a time lag of about1.5s for refilling the capillaries, indicating their function as capacitance vessels during RH.In conclusion, the coronary capillary network functions as 1) the major watershed between diastolic-dominant arterial and systolic-dominant venous flows, 2) a capacitor, and 3) a local flow amplifier and homogenizer of blood supply during RH.