This essay delineates the gradual process that in opera seria of the 1770s led to the formation of the typical formal scheme of the "rondò a due tempi", a model that later enjoyed widespread diffusion. In this period the predominant typology is still that of the vocal "rondò" in one movement: the slow passage was designed for the castrato at the moment of greatest dramatic pathos. A particular category of these "rondò" presents the principal theme in gavotte rhythm, and it is from this trend that the new rondò a due tempi is developed, as some of the early examples of the genre by Giuseppe Sarti demonstrate. The second movement of the "rondò", in "tempo veloce", arises as a simple coda of the old model in "tempo lento" in order to then expand and assume a most complex structural design, while conversely the slow part (deriving directly from the old type in one movement) is abbreviated. In these rondos, in both one and two "tempi", the morphology comes to assume a function previously unknown in the eighteenth-century "seria" aria: that of constructing a clear alternation between the lyric and static moment of the metastasian tradition (that of the "refrain") and a more dynamic section (the "couplet") in which the character interacts with the surrounding ambience while the "tempo drammatico" gushes forth. This juxtaposition in many ways anticipates the one between 'static' and 'kinetic' sections that will characterise the nineteenth-century 'solita forma'.