Recent research has emphasized the continuities in European republican political thought from the late Middle Ages until well into the Renaissance and even beyond. Two of the central figures in the story of the persistence of republicanism are Ptolemy of Lucca, who is commonly viewed as the quintessential late medieval republican, and Niccolò Machiavelli, whose work is generally regarded as the classic statement of early modern republicanism. We argue that these two remain conceptually at considerable remove from one another, a claim we illustrate by analyzing the impact of the reception, Latin translation and transmission of the Histories of Polybius, and especially the theory of constitutional change proposed in Book 6. The unavailability of the Histories to Ptolemy and its rather ample use by Machiavelli at the beginning of the Discourses signal an important divergence in the theoretical principles underlying the defense of republican institutions. In turn, this variation captures one facet of the distinct qualities of republican thought that separated the intellectual terrain of the early fourteenth century from that of the sixteenth century.