This study was designed to investigate the simultaneous changes in blood flow and microcirculation in an island flap during venous occlusion (venous ischemia), in an ischemia/reperfusion injury model in the rabbit. An island groin flap based on the inferior epigastric vessels was harvested in 15 rabbits. The flap was rendered ischemic for 3 hr (n = 5) or 4 hr (n = 10, 5 heparinized and 5 not), by applying a microvascular clamp to the inferior epigastric vein. Transonic Doppler and laser Doppler were used to monitor blood flow in the epigastric artery and microcirculation of the flap for 1 hr after flap elevation, 1 hr after occlusion, and for 3 hr at the end of the ischemic period. Venous occlusion was followed by a rapid decrease of blood flow and microcirculation readings. After ischemia, both blood flow and microcirculation readings in the flap were significantly decreased, compared to pre-ischemic values in all groups. In the 3-hr ischemia group, blood flow readings returned to pre-stress values, while microcirculation remained significantly lower. In the 4-hr ischemia group treated with heparin, blood flow in the artery settled at levels significantly lower than pre-stress readings; however, microcirculation of the flap was ultimately fully restored to pre-ischemic values. In the 4-hr ischemia group, both blood flow and microcirculation in the flap settled at levels significantly lower than pre-stress values. The authors concluded that tolerance for venous ischemia is time-dependent in this model and that venous ischemia is more deleterious than global ischemia. Administration of heparin may alter the time frame of ischemia/reperfusion injury and may prevent the harmful effects of injury at the microcirculatory level.