This article analyzes the livestock exchange between the United States and Mexico, beginning with the initial surge in regular trade in the 1870s until its interruption caused by the outbreaks of foot-and-mouth disease in 1947. Since the final two decades of the nineteenth century, cattle raising in both countries became increasingly intertwined through commerce. Though this trade was clearly international, as it entailed crossing a political border between nations, we argue that it was also an interregional commerce between contiguous, similar exporting bases, those of the southwestern United States and northern Mexico. However, the weight of politics and the legal border never ceased to make their presence felt and, indeed, were intensely manifested in the sphere of exchange. This particular branch of commerce was subject not only to numerous and repeated restrictive measures both related and unrelated to tariffs, but also to extraordinary political circumstances like the Mexican Revolution and the two World Wars. Our purpose is to reconstruct the interactions among resource endowments, commerce regulations, and supply and demand, as well as the actors and institutions that shaped the conditions of the transborder exchange. Such conditions allowed for continuity in livestock trading amid changing circumstances.