The combined gastrocnemius-soleus (G-S) muscle flap is a new research model for use in microvascular free tissue transfer in rats. Harvest of the combined G-S muscle flap is straightforward technically, and its increased size makes it particularly useful for biochemical and morphological study. A single G-S muscle complex accounts for approximately 1% of a rat's total body weight (261.67 +/- 21.60 g). It can be used as a free muscle flap based on its large femoral pedicle (artery diameter, 0.9 +/- 0.1 mm; vein diameter, 1.6 +/- 0.1 mm). Eight G-S muscle flaps were harvested as free muscle flaps and transferred to a recipient femoral site. The flaps were explored 24 hours after revascularization and assessed for viability by visual inspection of the muscle and vascular pedicle, as well as by fluorescein injection. A survival rate of 100% (8 of 8) was noted at 24 hours. This flap is reliable, easily harvested, and provides additional muscle bulk for microvascular research.