This article compares the transformation of carnival celebrations in Buenos Aires, Argentina and Montevideo, Uruguay at the beginning of the twentieth-century. It argues that changes in carnival practices in the River Plate region, a geography shared by these two national capitals, cannot be understood without taking a comparative stance since the demise of carnival in Buenos Aires was linked to the rise of a vacation culture in Montevideo. In addition, the more dramatic urbanization process experienced by Buenos Aires and the longer political stability of the nineteenth century here enabled elite politicians, commercial interests, and the middle and upper classes to regulate carnival behaviors to such an extent that the festivities became increasingly lackluster. In contrast, the slower urbanization process in Montevideo combined with the government's close but not overzealous regulation of carnival, allowed Montevideo's carnival to flourish. The disparate nature of Montevideo's carnival celebration that included the involvement of the city's diverse neighborhoods was also an important aspect of this carnival's popularity and success. This article also assesses the historiography on Montevideo's carnival which has cast a negative light on the impact of modernization and the festival. In comparison to Buenos Aires', carnival in Montevideo was fomented by governmental regulation. Finally, this article examines the relationship between carnival and each of the city's African-descent populations. [ABSTRACT FROM AUTHOR]