Using a method of vascular pedicle ligation without skin incision, some minor differences were found between the progress of neovascularization in microvascular free skin-fat flaps and island flaps, both with and without 1-hr ischemia in rats. Viable flap areas were established following vascular pedicle ligation (both vessels or only artery or vein), on the third day after flap replantation in both island and free flaps. Vascular pedicle occlusion on the fifth day after operation resulted in complete survival of free flaps only. Island flaps survived completely following vascular pedicle ligation on the seventh postoperative day. A clear correlation existed between the timing of vascular pedicle ligation and the necrosis and shrinkage rates. In free flaps, clinical signs of viability disturbances were seen earlier in single or total vascular pedicle occlusions, on the third or fourth day compared with island flaps. Insufficiency of the pedicle vein in island or free flaps was tolerated earlier than arterial or total pedicle occlusion. Venous occlusion three days after flap replantation caused significantly higher necrosis and shrinkage rates (p less than 0.05) in free flaps than in island flaps.