With respect to the arterial system, Leiopelma is typically Anuran in character and differs little except in minor details from even the more specialised types such as Rana and Hyla.

Leiopelma exhibits invariably persistent and well-developed right and left postcardinal veins. Larval forms of both urodeles and frogs possess these vessels, but in the adult they are usually replaced by a median postcaval vein. Some adult urodeles retain postcardinal veins as well as a postcaval vein, and a similar condition is found in a few of the less specialised Anurans. Postcardinals are found in Ascaphus, the only other member of the Leiopelmidae (Noble, 1931), but their condition has apparently not been described in detail. Certain Discoglossid types—e.g., Bombinator igneus, B. bombinus and Discoglossus pictus possess small postcardinal veins, and these vessels have also been found occasionally in Alytes obstetricans (Howes, 1888). Xenopus laevis, among the Pipidae, and various species among the more specialised families such as the Ranidae and Bufonidae show an occasional irregular occurrence of one or both postcardinals. The main literature regarding these types is summarised by O'Donoghue (1931 and 1933). Whatever may be the reason for the occasional persistence of postcardinals in certain types, their invariable presence in Leiopelma and Ascaphus clearly supports the evidence already known regarding the primitive nature of this family.