In order to explain the variability noted in the fluorescein staining characteristics of the epigastric axial flap in the experimental laboratory rat, the anatomy of the blood supply of this particular flap was investigated. In 10 rats, the vascular pattern of the superficial epigastric trunk to the abdominal skin was found to be similar. A small branch of the trunk extended toward the medial abdominal skin to collateralize with a branch of the internal mammary vessel entering from the chest skin. A large extension of the superficial epigastric trunk extended laterally toward the midlateral point of the animal and cephalad to collateralize with the vessels in the lateral chest wall, presumably the lateral thoracic artery. Fluorescein staining characteristics of 10 flaps including or excluding the lateral branch were examined, and a significant difference was found in the immediate fluorescein staining characteristics. The flaps that included the lateral branch fluoresced to an average of 89.2 percent of the flap length, and those in which the lateral branch was excluded fluoresced to 68.7 percent of the flap length. The use of this flap model in experimental flap research should include a precise description of the anatomy of the particular flap used in order to ensure reproducible results.