The pathway of venous drainage in retrograde island flaps was investigated by fluorescence tracing technique using the saphenous fasciocutaneous flap in New Zealand White rabbits. Forty animals were allocated into four groups according to the different times at 30 minutes (I), 24 hours (II), 72 hours (III), and 7 days (IV) after the operation. According to the different routes to give tracer, each group was further allocated into two subgroups of the artery injection and vein injection. For each animal, one hindlimb was assigned as the experimental side, the contralateral side as control without giving tracer. The erythrocytes were separated, labeled with fluorescein isothiocyanate (FITC), detected, and injected into the artery or vein. Subsequently, the flaps were harvested 5 seconds after injection and immediately frozen, sectioned, and observed under microscope. In group I and II, the fluorescence was observed mainly around the vessel adventitia of the vein and artery and tunica intima of the artery. In group III, there was weak fluorescence observed in the lumen of vein. In group IV, fluorescence was distributed principally in the lumen of the vein. In addition, fluorescence was not observed in the saphenous nerve in group I and there was mild fluorescence in the saphenous nerve in groups II, III, and IV. These findings suggest that the venous return is through “bypass route” in earlier period. In later period, the venous retrograde return is through “bypass route” and “incompetent valves route;” however, “incompetent valves route” becomes the main route. © 2009 Wiley-Liss, Inc. Microsurgery 2010.