This study investigated the microcirculatory pattern during acute rejection of a vascularized free muscle graft. Using a new rat cremaster muscle free flap model, isograft transplantation was done between genetically identical Lewis rats (N = 23) and allograft transplantation across a major histocompatibility barrier (Lewis Brown Norway [RT-11 + n] to Lewis [RT-11] rats; N = 24). At days 1, 3, 5, and 7 posttransplantation, microcirculatory measurements were taken of vessel diameter, red blood cell velocity, endothelial edema index, leukocytic-endothelial interaction (rolling, adhering, and transmigrating leukocytes), and capillary perfusion. In the allografts at 5 days the lumen obstruction was 27% greater (p < 0.05) than onset. The number of perfused capillaries decreased by 36% compared with isografts at day 5 (p < 0.05). The number of adhering leukocytes increased significantly in allografts both at day 1 (71%; p < 0.05) and at day 3 (77%; p < 0.05). At day 3, transmigrating leukocytes rose significantly (p < 0.05). Allograft histology showed myocytic destruction and lymphocytic infiltration at day 5. Our results suggest that the microcirculatory signs of acute muscle allograft rejection occurred 24 hours after transplantation, and preceded histological signs of rejection. The critical time for microvascular survival of muscle allografts was 3 days. This procedure allows us to distinguish events related to transplantation trauma from rejection phenomena.