The purpose of this study was to describe a new musculocutaneous flap model in the rat. A total of 25 Wistar rats weighing 200 to 280 g were used in this experiment. In 15 rats, the vascular anatomy of the biceps femoris muscle and the cutaneous blood supply of its overlying posterior thigh skin were studied by anatomic dissection, dye injection, and microangiography using 5 rats in each group. The anatomic studies revealed that the main axial vessel supplying the biceps femoris muscle was the caudal femoral branch of the popliteal vessels. The posterior thigh skin overlying the biceps femoris muscle received a consistent musculocutaneous perforator at the center of the mid-posterior line of the posterior thigh. Based on the caudal femoral-popliteal vascular pedicle, the biceps femoris musculocutaneous flap was created in the rat, comprised of the whole muscle and its overlying posterior thigh skin. The skin paddle was designed as an ellipse with its longitudinal axis paralleling that of the extremity, generally measuring 4 x 2 cm. Island flaps were raised as described and replaced either in situ (N = 5) or transposed to a sacral defect(N = 5). Results showed that the cutaneous islands of all the flaps survived completely. Tetrazolium blue stain used to indicate muscle survival revealed that the average muscle viability was 86.7 +/- 3.4%. The authors conclude that the biceps femoris musculocutaneous flap is a reliable and true musculocutaneous flap model for future biological and pharmacological studies. It offers the following advantages: It has a consistent vascular pedicle and a musculocutaneous perforator, it supports a relatively large skin island, and there is no risk of autocannibalization of the flap because the flap is located dorsally.