The ontogeny of the arteries of the pelvic extremity of Tupaia belangeri was investigated by light microscopy on the basis of serial sections of 30 embryos, dating from day 17 to day 42 post-copulation. In Tupaia, the gestational period takes approximately 43 days. Additionally, a 3-D reconstruction of the pelvic region and the right leg of a 22-day embryo was prepared. The arteries of an adult Tupaia were studied on the basis of a corrosion cast. The results were compared with the ontogeny of the arterial system of other mammals. In the 17-day embryo, the anlage of the pelvic extremity is penetrated by a capillary plexus. In the 18-day embryo, the a. ischiadica reaches the pelvic limb bud, representing the primary axial artery. On day 19, its r. perforans tarsi extends from the plantar to the dorsal aspect of the foot plate. The a. ischiadica is the main artery of the leg until the stage of the 22-day embryo. Afterwards, the peripheral arteries supplied by it are taken over by the a. iliaca externa and its extension, the a. femoralis. The a. iliaca externa springs from the a. iliaca communis in the 19-day embryo. From day 21 to day 22, the capillary plexus, which is nourished by the a. femoralis, closely approaches the a. ischiadica, and finally, a connecting branch joins the a. ischiadica. The a. ischiadica is then reduced to the a. glutea caudalis, and the aa. femoralis, poplitea profunda (at the cranial aspect of the m. popliteus), and interossea become the main arteries of the pelvic extremity. The a. poplitea superficialis, lying at the caudal aspect of the m. popliteus, and its continuation in the crural region, the a. peronea, develop until the 25-day embryo. The a. peronea gives rise to an r. perforans which penetrates the membrana interossea towards the dorsum of the foot. As a result of a shift of the origin of the a. iliaca externa in the proximal direction, the length of the a. iliaca communis gradually decreases until, on day 24, the a. iliaca externa springs directly from the lateral wall of the aorta. In the 20-day embryo, the a. iliaca externa gives rise to an a. circumflexa ilium profunda towards the lateral pelvic wall, and in 23-day embryos, to the a. profunda femoris. The main branches of the a. profunda femoris develop until day 24. At the same time, the aa. circumflexa femoris lateralis and nutricia ossis femoris arise from the a. femoralis. The a. saphena, which is already recognizable in the 23-day embryo, gives rise to the a. genus descendens, and as an a. plantaris medialis, to four aa. digitales plantares communes (I-IV) at the planta pedis. The development of the a. tibialis cranialis on day 25 takes place independently and without any topographic relation to the a. saphena, which functionally replaces the a. tibialis cranialis in some other mammals. In the 26-day embryo, the aa. peronea and tibialis cranialis extend to the dorsum of the foot where they continue as the aa. dorsales pedis profunda and superficialis. The fourth main artery of the lower leg, the a. caudalis femoris, which is first observed in the 20-day embryo, reaches the lateral aspect of the foot on day 24. Its r. calcaneus runs to the planta pedis. In 30-day embryos, the aa. digitales plantares propriae have differentiated. The corresponding dorsal arteries and the superficial plantar vascular are develop until day 35, so that all important arteries of the pelvic extremity, which are seen in the corrosion cast of the adult, are recognizable. Among the embryos and the adult Tupaia studied, individual variation is minimal. The developmental stage at which the arteries of the leg acquired a secondary vascular wall was ascertained. Only a vessel with a primary vascular wall can dissolve into a capillary plexus later on (e.g., a. interossea). In contrast, the course of an artery which has acquired a secondary vascular wall is determined, because modifications of the course of a vessel often need a capillary plexus as an intermediate st